Praise be to Allah.
It is mustahabb for the Muslim to remember Allaah a great deal (dhikr), because Allaah says (interpretation of the meaning):
“O you who believe! Remember Allaah with much remembrance.
42. And glorify His Praises morning and afternoon”
“…and the men and the women who remember Allaah much with their hearts and tongues. Allaah has prepared for them forgiveness and a great reward (i.e. Paradise)”
And there are many similar verses. And the Prophet (peace and blessings of Allaah be upon him) also encouraged us to do that, when he said: “Keep your tongue moist with the remembrance of Allaah.” Narrated by al-Tirmidhi and classed as saheeh by al-Albaani in Saheeh al-Tirmidhi, 2687. The dhikrs that were narrated from the Messenger (peace and blessings of Allaah be upon him) are of two types:
1 – Those for which a specific number has been narrated, in which case it is obligatory to keep to the number narrated from the Prophet (peace and blessings of Allaah be upon him), no more and no less – such as the dhikrs to be recited following the prayer, or saying Subhaan-Allaah wa bi hamdih (glory and praise be to Allaah) one hundred times morning and evening, or saying Laa ilaaha ill-Allaah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd, wa huwa ‘ala kulli shay’in qadeer (There is no god but Allaah alone, with no partner or associate; His is the Dominion, all praise be to Him, and He is Able to do all things) one hundred times each day, and so on.
2 – Those for which no specific number has been narrated, such as the general encouragement to say tasbeeh (‘Subhaan-Allaah’ – Glory be to Allaah), tahmeed (‘Al-hamdu Lillaah’ - Praise be to Allaah) and tahleel (‘Laa ilaaha ill-Allaah’ – There is no god but Allaah), and to recite Qur’aan and so on. This may be done in general terms, without keeping to any specific number, as prescribed by the Prophet (peace and blessings of Allaah be upon him).
It says in Fataawa al-Lajnah al-Daa’imah:
The basic principle with regard to dhikr and acts of worship is tawqeef (i.e., adhering to what is mentioned and prescribed in the Qur’aan and Sunnah and not worshipping Allaah in any way except that which has been prescribed on the lips of the Messenger of Allaah (peace and blessings of Allaah be upon him)). Based on this, in cases where a certain time, number of times, place or manner has been specified, we must worship Allaah in the way that has been proven in sharee’ah. But when Allaah has prescribed dhikrs, du’aa’ and other kinds of worship in general terms without specifying a particular time, number, place or manner, then it is not permissible for us to set limits with regard to the manner, time or number of times. Rather we should worship Him in general terms as it was narrated.
From: Fataawa Islamiyyah, 4/178.
And Allaah knows best. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions.