Muwatta Malik Book 7, Hadith 18
Yahya related to me from Malik that he had heard that A'isha, the
wife of the Prophet, may Allah bless him and grant him peace, used to
say, "If someone fears that he will sleep through till the morning,
let him pray the witr before he sleeps, and if some one hopes to wake
for the last part of the night, let him delay his witr."
Yahya related to me from Malik that he had heard that a man asked Abdullah ibn Umar whether the witr was obligatory and Abdullah ibn Umar said, "The Messenger of Allah, may Allah bless him and grant him peace, prayed witr, and the muslims prayed witr." The man began repeating his question, and Abdullah ibn Umar kept saying, "The Messenger of Allah, may Allah bless him and grant him peace, prayed witr, and the muslims prayed witr."
Yahya related to me from Malik that Nafi said, "I was with Abdullah ibn Umar in Makka. The sky was clouded over and Abdullah feared that dawn was approaching so he prayed one raka for witr. Then the clouds cleared and he saw that it was still night, so he made his prayers even with one raka. Then he continued to pray two rakas at a time, until when he feared the approach of dawn, he prayed one raka for witr."