solat tahajjud

The Best time for Tahajjud

The best time to perform it at the last third portion of the night as Abu Hurairah (radiallāhu`anhu) quoted that the Messenger of Allāh (Sallallāhu 'alayhi wa sallam), as saying: "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?" [Narrated by Ahmad, al-Bukhāri, and an-Nasā'ie]

‘Amr ibn Abasah (radiallāhu`anhu) reported that he heard the Prophet (Sallallāhu ‘alayhi wa sallam) saying: "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 Allāh, the Exalted One, at that time then does so." [Narrated by Ahmad, al-Bukhari, and an-Nasā'ie]

Abdullāh ibn 'Amr (radiallāhu’anhu) reported Rasūlullāh (Sallallāhu 'alayhi wa sallam)  as saying: With Allāh the best fasting is that of Dawud and the best prayer is that of Dawud (‘alaihissalam) for he slept half of the night and stood for prayer for the third of it and (then) slept the sixth part of it and he observed fast one day and broke on the other. [Narrated by Muslim (2595)]

The number of raka`at to be performed during Tahajjud

Solāt Tahajjud does not entail a specific number of raka`at which must be performed nor is there any maximum limit which has to be performed. Aishah (radiallāhu`anha) narrated that the number of raka’at  was not fixed by Rasūlullāh (sallallāhu 'alayhi wa sallam) but rather a changeable according to the situation of his health, or whether it was done in standing or sitting form. 

Aishah (radiallāhu`anha) narrated Rasūlullāh (Sallallāhu 'alayhi wa sallam) (sallallāhu 'alayhi wa sallam) used to perform eleven raka’at, in form of an extended two four-raka’at [eight raka’at of Tahajjud], then perform three-raka’at [of Solat Witir.] [Bukhari, Muwatta, Muslim], and in some narrations, he used to perform it in the raka’at-of-twos, which is mustahab way, as Ibn 'Umar (radiallāhu`anhu) reported that a companion asked the Messenger of Allāh (Sallallāhu 'alayhi wa sallam) about the night prayer. The Messenger of Allāh (Sallallāhu 'alayhi wa sallam) said: "In sets of two, in sets of two. Then when one of you fears the coming of morning, he prays one (raka'ah) that will serve as a Solat Witir (i.e. make the number odd) of what he has prayed." [Al-Bukhari no. 990 and Muslim no. 749] Many narrations, Rasūlullāh (sallallāhu 'alayhi wa sallam) would begin the tahajjud with a two short raka’at [Muslim, Abu Dawud]. And in some narrations, Tahajjud Prayer is mentioned only as a two raka’at [Bakhari]. And it is narrated that this number lessened to 9 and 7 as his age progressed [Bukhari, Muslim, and Abu Dawud]

source

http://al-hussaini.blogspot.com/2011/09/tahajjud-virtues-and-way-of-performance.html