All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Tahajjud is the late night prayer performed after a short sleep. Its excellence and reward are more than any other voluntary prayer and it has a good effect on the praying person himself. Imam Haakim narrated from Abu Umaamah al-Baahili that the Prophet Muhammad, sallallaahu ‘alayhi wa sallam , said: “Observe the night prayer; it was the practice of the righteous before you and it brings you closer to your Lord; it is penance for evil deeds and (it) erases sins.” In addition, Imaam Muslim narrated from Abu Hurayrah that the Prophet Muhammad, sallallaahu ‘alayhi wa sallam, said: “The most excellent prayer after what is prescribed is prayer during the night."
The best time for Tahajjud is the last-third of the night. Imaam At-Tirmithi narrated from ‘Amr Ibn ‘Absah that he said, “O Prophet, which part of the night is best for Allaah to accept our supplication?” He said: "The middle of the latter half of the night. So pray (in it) whatever you wish (to pray)."
It is disliked for anyone who has gotten used to performing Tahajjud to then stop doing it without a legal reason. Prophet Muhammad, sallallaahu ‘alayhi wa sallam, said to Abdullaah Ibn ‘Umar : "O Abdullaah, do not become like so-and-so who used to make the Tahajjud prayers and then he stopped praying it." [Al-Bukhari]
Prophet Muhammad, sallallaahu ‘alayhi wa sallam, prayed eleven Rak'ahs (units) and also thirteen, as reported in sound Hadeeths. There is no harm if one increases the number of Rak’ahs or decreases them, but the important thing to bear in mind is that the constancy of doing something is far better than doing much work without constancy. Aa’ishah said that the Prophet Muhammad, sallallaahu ‘alayhi wa sallam, said: “The deed that Allaah likes most is the one that is performed regularly, no matter how small it is.”
Allaah Knows best.