Tahajjud is the voluntary night prayer performed after sleeping for at least part of the night, distinguishing it from other night prayers like Isha or witr prayed before sleep. The Quran commands the Prophet, 'And from part of the night, pray with it as additional [worship] for you; it is expected that your Lord will resurrect you to a praised station' (17:79). Its virtue is tied to the special nearness of Allah in the later hours: the Prophet said that Allah descends to the lowest heaven in the last third of every night, asking who will call upon Him so that He may answer (Sahih al-Bukhari 1145). There is no fixed number of rak'ahs; it is prayed in pairs of two, ending in witr, and can range from as few as two rak'ahs to many more depending on one's stamina and how much of the Quran one wishes to recite. While obligatory only upon the Prophet himself according to most scholars, it remains one of the most highly recommended acts of worship, especially valued in the last third of the night when distractions are fewest and sincerity is easiest to attain.
Q&A · Prayer
What is tahajjud prayer, and how and when is it performed?
References
17:79Sahih al-Bukhari 1145
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.