Firstly:
Reciting Quran all together in unison is not prescribed, because it is not mentioned in the hadith reports, and the basic principle regarding acts of worship is that they are restricted to what is narrated in authentic texts, as only authentic texts can confirm that a certain act of worship is prescribed in terms of the act, the way it is done, the time and place in which it is to be done, and the number of times a particular action is to be repeated. So an act of worship cannot be done in any particular way except on the basis of evidence.
Ash-Shatibi (may Allah have mercy on him) said: Thus innovation (bid`ah) refers to an invented way of religious practice that is similar to what is prescribed, but the intention in doing it is to go to extremes in worshipping Allah, may He be Exalted… That includes certain ways of doing it and certain circumstances for doing it, such as reciting dhikr all together in unison, or taking the day of the Prophet’s birth as an eid (festival), and so on.
It also includes committing oneself to doing particular acts of worship at a particular time for which there is no religious text to support that, such as committing to fast on the fifteenth day of Sha`ban (an-nisf min sha`ban) and spending the night before in prayer (qiyam).
End quote from Al-I`tisam, 1/37-39.
If the recitation is done out loud in such a manner that it disturbs other people who are sitting and remembering Allah (dhikr), then it is more disliked, because of the general meaning of the words of the Prophet (blessings and peace of Allah be upon him): “When one of you stands in prayer, he is conversing with his Lord, so let one of you know what he is saying when he converses with his Lord, and do not raise your voices over one another when reciting in prayer.”
Narrated by Ahmad, 4928; Shu`ayb al-Arna’ut classed it as authentic in Tahqeeq al-Musnad.
But if one of the group recites and the others listen to his recitation, or they all take turns to recite, or the group is in the mosque and each of them recites to himself without disturbing his neighbour or reciting the same words at the same time, there is nothing wrong with that; rather it is a prescribed act of worship that is beloved to Allah, may He be Exalted.
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “…No people gather in one of the houses of Allah, reciting the Book of Allah and studying it together, but tranquillity will descend upon them, mercy will overshadow them, the angels will surround them and Allah will mention them to those who are with Him.” Narrated by Muslim, 2699.
Imam an-Nawawi (may Allah have mercy on him) said:
Section: the recommendation for a group to recite together, the virtue of those who recite Quran and those who listen, and highlighting the virtue of the one who brings them together and urges and encourages them to attend. Then he said: You should understand that it is recommended to come together to recite Quran because of the clear evidence and the actions of the earlier and later generations which indicate that it is a good thing to do. Then he quoted the hadith mentioned above and similar texts.
See: At-Tibyan fi Adab Hamalat al-Quran, 72-74.
Then after that he wrote a section on taking turns to recite Quran, in which he said: This refers to a group who come together, and one of them recites one tenth of a juz’ or a juz’ or another portion of Quran, then he falls silent and someone else recites, carrying on from where the first one stopped, then another one recites, and so on. This is permissible and good. Malik (may Allah have mercy on him) was asked about that and he said: There is nothing wrong with it.
End quote from At-Tibyan, 74.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Gathering to remember Allah, listen to His Book, and offer supplication (du`a’) is a righteous deed, and it is one of the best acts of worship at all times. In As-Sahih it is narrated from the Prophet (blessings and peace of Allah be upon him) that he said: “Indeed, Allah has angels who go about on the earth and when they find people remembering Allah, they call to one another: ‘Come to that which you have been seeking.’” And he quoted the hadith, in which it says: “ ‘We found them glorifying You and praising You.’”
But this should only be done at occasionally and in different places, so that it does not become a regular practice to which people are committed, except for what the Messenger of Allah (blessings and peace of Allah be upon him) made a prescribed practice that is to be done regularly in congregation, namely the five daily prayers, Jumu`ah prayer, Eid prayer and so on.
As for an individual’s regular practice of acts of worship (wird), such as praying, reciting Qur’an, remembering Allah (dhikr) or offering supplication (du`a’) at the two ends of the day and in some hours of the night, and so on, this is the practice of the Messenger of Allah (blessings and peace of Allah be upon him) and the righteous slaves of Allah both in the past and at present.
So whatever is prescribed to be done in congregation, like the obligatory prayers, should be done in congregation, and whatever is prescribed to be done consistently on one’s own, such as dhikr and supererogatory acts of worship should be done on one’s own. The Sahabah (may Allah be pleased with them) would sometimes gather and would tell one of their number to recite Quran whilst the others listened. `Umar ibn al-Khattab (may Allah be pleased with him) would say: O Abu Musa, remind us of our Lord, and he would recite whilst they listened. Some of the Sahabah would say: Let us sit down and increase our faith for a while. The Prophet (blessings and peace of Allah be upon him) led his Companions in offering a voluntary prayer in congregation several times. He came out to the Sahabah – the Ahl as-Suffah – when one of them was reciting, and he sat down with them to listen.
End quote from Majmu` al-Fatawa, 22/521.
It says in Fatawa al-Lajnah ad-Da’imah, 4/112: What is the ruling on reciting Quran together in the mosque?
Answer: The question is ambiguous. If what is meant is that they recite all together in unison, reciting and pausing at the same time, that is not prescribed, and the least that can be said about it is that it is disliked (makruh), because it is not narrated that the Messenger of Allah (blessings and peace of Allah be upon him) or the Sahabah (may Allah be pleased with them) did that. But if that is done for the purpose of teaching, then we hope that there is nothing wrong with it.
If what is meant is that they gather to recite Quran for the purpose of memorizing it or learning it, so one of them recites whilst the others listen, or each of them recites by himself, not matching others in recitation or reciting along with others, then this is something that is prescribed, because it was soundly narrated from the Messenger of Allah (blessings and peace of Allah be upon him) that he said: “…No people gather in one of the houses of Allah, reciting the Book of Allah and studying it together, but tranquillity will descend upon them, the angels will surround them, mercy will overshadow them, and Allah will mention them to those who are with Him.” Narrated by Muslim. End quote.
It also says (2/480): Gathering to recite Quran and study it, where one person recites and the others listen, and they study together what they recite and learn about its meanings, is something that is prescribed and is an act of worship that Allah loves and will grant an immense reward for it. It was narrated by Muslim in his Sahih and by Abu Dawud from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “No people gather in one of the houses of Allah, reciting the Book of Allah and studying it together, but tranquillity will descend upon them, mercy will overshadow them, the angels will surround them and Allah will mention them to those who are with Him.” End quote.
Secondly:
The prostration of recitation is Sunnah for both the reciter and the listener, but not the one who happens to hear it but was not listening or intending to listen.
Ibn Qudamah (may Allah have mercy on him) said in Al-Mughni, 1/361: It is Sunnah for both the reciter and the listener to prostrate, and we do not know of any difference of scholarly opinion concerning that. This is indicated by the hadiths that we narrated. Al-Bukhari, Muslim and Abu Dawud narrated that Ibn `Umar said: The Messenger of Allah (blessings and peace of Allah be upon him) would recite a surah to us at times other than prayer, and he would prostrate and we would prostrate with him, until one of us could not find a place to put his forehead.
As for the one who hears it without intending to listen, it [the prostration of recitation] is not recommended in his case. That was narrated from `Uthman, Ibn `Abbas and `Imran, and it was the view of Malik. However, as-hab ar-ra’y said that he should prostrate, and something similar was narrated from Ibn `Umar, an-Nakha`i, Sa`id ibn Jubayr, Nafi` and Is-haq, because he can hear the verse in which there is a sajdah (prostration), so he should prostrate, like the one who is listening. And ash-Shafa`i said: I do not insist that he should prostrate, but if he prostrates, it is good.
Our view is supported by what was narrated from `Uthman (may Allah be pleased with him), that he passed by a preacher, and the preacher recited a verse in which there is a sajdah so that `Uthman would prostrate with him, but he did not prostrate, and he said: The prostration is only required of the one who is listening. Ibn Mas`ud and `Imran said: We did not sit down for the purpose of listening. Salman said: We did not come here for that purpose [that is, listening to the preacher]. Something similar was narrated from Ibn ‘Abbaas. No one differed from them at their time as far as we know, apart from what Ibn `Umar said: The prostration is only required of one who hears it. It may be that he meant one who listens deliberately, so what he said should be interpreted in this way so as to reconcile between their views. It is not right to compare one who happens to hear the recitation to one who is actively listening to it, because there is a difference in their rewards. End quote.
And Allah knows best.