Sahih Al Bukhari Hadith Number 1137

Narrated `Abdullah bin `Umar: A man said, “O Allah’s Messenger (peace be upon him)! How is the prayer of the night?” He said, “Two rak`at followed by two rak`at and so on, and when you apprehend the approaching dawn, offer one rak`a as witr.”

19  Prayer at Night (Tahajjud)
Hadith number 1137
Volume no 2
Chapter: 10 (How was the Salat of the Prophet (peace be upon him) and how many Rak’a he used to offer at night?)