Praise be to Allah.
The practice of the Prophet (blessings and peace of Allah be upon him) in most instances was to make Fajr prayer lengthy, make ‘Isha’ of moderate length, and make Maghrib brief, as an-Nasaa’i (982) narrated from Sulaymaan ibn Yasaar from Abu Hurayrah, who said: I never prayed behind anyone whose prayer most closely resembled that of the Messenger of Allah (blessings and peace of Allah be upon him) than So and so. Sulaymaan said: He used to make the first two rak‘ahs of Zuhr lengthy and make the last two brief; he would make ‘Asr brief; in Maghrib he would recite the short mufassil soorahs; in ‘Isha’ he would recite the medium mufassil soorahs; and in Fajr he would recite the lengthy mufassil soorahs.
Classed as saheeh by al-Albaani in Saheeh an-Nasaa’i.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
The lengthy mufassil soorahs are from Qaaf to the beginning of an-Naba’. The medium mufassil soorahs are from an-Naba’ to ad-Duha. The short mufassil soorahs are from ad-Duha to the end of the Qur’an.
End quote from ash-Sharh al-Mumti‘ (3/75).
Ibn Battaal (may Allah have mercy on him) said: The scholars are agreed that the longest of the prayers in terms of recitation is Fajr.
End quote from Sharh Saheeh al-Bukhaari (2/385).
The Prophet (blessings and peace of Allah be upon him) did not regularly recite any particular soorahs in Fajr prayer, except on Fridays, when he would recite Alif-Laam-Meem Tanzeel (Soorat al-Sajdah) and Hal ata ‘ala al-insaan (Soorat al-Insaan). Narrated by al-Bukhaari (891) and Muslim (880).
On days other than Friday, sometimes he would recite al-Waaqi‘ah and similar soorahs in Fajr prayer. Narrated by Ahmad (21033); classed as saheeh by al-Albaani in Sifat as-Salaah (p. 109).
On one occasion he recited at-Toor. Narrated by al-Bukhaari (1626) and Muslim (1276).
Occasionally he recited Qaaf wa’l-Qur’an il-Majeed (Soorat Qaaf). Narrated by Muslim (458).
On one occasion he recited in Fajr prayer Idha zulzilat (Soorat az-Zalzalah) in both rak‘ahs. Narrated by Abu Dawood (816); classed as saheeh by al-Albaani in Saheeh Abi Dawood.
On one occasion he recited in Fajr prayer whilst travelling Qul a‘oodhu bi Rabb il-Falaq (Soorat al-Falaq) and Qul a‘oodhu bi Rabb in-Naas (Soorat an-Naas). Narrated by Abu Dawood (1462); classed as saheeh by al-Albaani in Saheeh Abi Dawood.
On one occasion he prayed Fajr in Makkah and began reciting Soorat al-Mu’minoon until he came to the verse that mentions Moosaa and Haroon, then he (blessings and peace of Allah be upon him) was overcome by a cough, so he bowed. Narrated by Muslim (455).
On one occasion he led them in praying Fajr and recited Soorat as-Saaffaat. Narrated by Ahmad (4989); classed as saheeh by al-Albaani in Sifat as-Salaah (p. 109).
Abu Barzah (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) used to pray Fajr and one of us would be able to recognise the person next to him (because it grew very light), and he would recite between sixty and one hundred verses. Narrated by al-Bukhaari (541) and Muslim (461).
We have not come across any report that says that the Prophet (blessings and peace of Allah be upon him) used to recite in Fajr prayer or any other prayer the beginning of Soorat al-Kahf and the end of Soorat al-Jumu‘ah.
What the imam does of singling out these two soorahs in Fajr prayer appears to be contrary to the Sunnah of the Prophet (blessings and peace of Allah be upon him) and his usual practice in prayer. In fact it appears more likely to be a kind of bid‘ah (innovation). Shaykh Bakr Abu Zayd (may Allah have mercy on him) said: It is an innovation to single anything out without evidence to that effect.
End quote from Bida‘ al-Qiraa’ah (p. 14).
The scholars of the committee were asked:
In Fajr prayer I always recite Soorat al-Inshiraah in the first rak‘ah, because I believe and feel when I recite this particular soorah that I am beginning a new day. Please note that I have memorised a lot of the soorahs of the Holy Qur’an. What is the ruling on singling out a particular soorah – namely Soorat al-Inshiraah – for the first rak‘ah of Fajr prayer?
what is prescribed in Fajr prayer is to make the recitation lengthy, and to recite whatever one can of Qur’an after al-Faatihah, without singling out a particular soorah, except in cases where a particular soorah has been mentioned in reports, such as Soorat as-Sajdah and Soorat al-Insaan in Fajr prayer on Fridays, because singling out a soorah without evidence to that effect comes under the heading of innovation.
End quote from Fataawa al-Lajnah ad-Daa’imah (5/340)
What this imam should do is be keen to follow the Sunnah in his prayers and when leading people in prayer. He has not been appointed to this position to lead them in a manner that he likes and prefers; rather what he must do is follow the practice of the Prophet (blessings and peace of Allah be upon him) in his prayer, and he should strive hard to follow his Sunnah and do what is recommended in prayer, as much as the people behind him can cope with.
Fajr prayer is a prayer in which recitation is done out loud, in which people – especially nowadays – need to hear the words of Allah, so varying what is recited is better and is more beneficial to people, brings greater blessing and is more likely to avoid boredom, because by doing that people will hear a lot of Qur’an, especially those who do not read it well or who are distracted from the Qur’an by work and so on.
And Allah knows best.