Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.025.

It is best to delay this prayer to the last third portion of the night. Abu Hurairah reports that the Messenger of Allah said: “Our Lord descends to the lowest heaven during the last third of the night, inquiring: ‘Who will call on Me so that I may respond to him? Who is asking something of Me so I may give it to him? Who is asking for My forgiveness so I may forgive him?”‘ This is related by the group.

‘Amr ibn Abasah reports that he heard the Prophet say: “The closest that a slave comes to his Lord is during the middle of the latter portion of the night. If you can be among those who remember Allah, the Exalted One, at that time then do so.” This is related by al-Hakim who grades it sahih according to Muslim’s standards, and at-Tirmizhi calls it hasan sahih. AnNasa’i and Ibn Khuzaimah also recorded it.

Abu Muslim asked Abu Zharr: “Which  late-night prayer is the best?” He said: “I asked the Messenger of Allah the same that you asked me and he said, ‘The (one done during) middle of the latter half of the night, and very few do it.'” This is related by Ahmad with a good chain.

‘Abdullah ibn ‘Amr reports that the Prophet said: “The most beloved fast to Allah is the fast of David. And the most beloved prayer to Allah is the prayer of David. He would sleep half of the night and then pray during the next third of the night and then sleep during the last sixth of the night. And he would fast one day and not fast the next.” This is related by the group except at-Tirmizhi.

The number of rak’at to be performed during tahajjud: The tahajjud prayer does not entail a specific number of rak’at which must be performed nor is there any maximum limit which has to be performed. It would be fulfilled even if one just prayed one rak’ah of witr after the obligatory night prayer.

Samurah ibn Jundub says: “The Messenger of Allah ordered us to pray during the night, a little or a lot, and to make the last of the prayer the witr prayer.” This is related by at-Tabarani and al-Bazzar.

Anas relates that the Messenger of Allah said: “Prayer in my mosque is equal to ten thousand prayers [elsewhere]. And prayer in the inviolable mosque is equivalent to one hundred thousand prayers [elsewhere]. And prayer in the battlefield is equivalent to one million prayers [elsewhere]. And what is more than all of that is two rak’at by a slave [of Allah] during the middle of the night.” This is reported by Abu ash-Shaikh, Ibn Hibban in his work ath-Thawab, and al-Munzhiri, in his book at-Targhib watTarhib, is silent about it.

Iyas ibn Mu’awiyyah al-Mazni reports that the Prophet sallallahu alehi wasallam said: “The night prayer should certainly be performed even if it is for the length of time that it takes one to milk a sheep. And whatever is after the obligatory ‘isha is of the tahajjud.” This is related by at-Tabarani, and all of its narrators are trustworthy save Muhammad ibn Ishaq.

Ibn ‘Abbas relates: “I mentioned the tahajjud prayer and some of the people said that the Prophet sallallahu alehi wasallam said: ‘It may be half of the night, a third of the night, a fourth of the night or a fraction of the time for milking a camel or a sheep.””

Ibn ‘Abbas also narrates that the Prophet ordered them and encouraged them to make the tahajjud prayer to the extent that he said: “You should perform salatul layil even if it is just one rak’ah.” This is related by atTabarani in al-Kabir and al-Awsat.