From Sahih Bukhari (Hadith 1137):
Narrated `Abdullah bin `Umar:A man said, "O Allah's Messenger (ﷺ)! 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
Reference: Bukhari 1137