‘Ali said that God’s Messenger used to pray three rak'as when observing the witr, reciting nine suras from al-Mufassal in the course of them, three in each rak'a, the last of them being “Say, He is God, One” (Al-Qur’an; 112).
Malik said he heard that a man asked Ibn ‘Umar whether the witr was obligatory, and ‘Abdallah said, “God’s Messenger observed the witr and the Muslims observed the witr.” The man kept repeating his question and ‘Abdallah kept saying, “God’s Messenger observed the witr and the Muslims observed the witr.”
Nafi' said, “I was with Ibn ‘Umar in Mecca when the sky was cloudy and he was afraid the morning might have come, so he observed a witr with one rak'a. The sky then cleared and he saw that it was still night, so he made it a pair by praying one rak'a; then he prayed series of two rak'as, and when he was afraid morning was near he observed a witr with one.”
Abu Sa'id reported God’s Messenger as saying, “If anyone over-sleeps and misses the witr, or forgets it, he should pray when he remembers and when he awakes.”