Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain (it is narrated from Sayyidah Aisha, may Allah be pleased with her) that the Messenger of Allah, peace and blessings be upon him, used to pray eleven rak'ahs at night. Among them, he would perform one witr (at the end). When he finished this, he would lie down on his right side.

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that a man asked the Messenger of Allah (peace and blessings be upon him) about the night prayer. The Messenger of Allah (peace and blessings be upon him) said: "The night prayer is in sets of two rak'ahs. Then when one of you fears that morning is approaching, he should pray one rak'ah, and that will make what he has prayed an odd number (witr) for him."

It is narrated from Ibn Muhairiz (may Allah have mercy on him) that a man from Banu Kinanah, Mukhdaaji, heard Abu Muhammad in Syria saying that Witr is obligatory. Mukhdaaji said: Then I went to Sayyiduna Ubadah bin As-Samit (may Allah be pleased with him). When I came face to face with him, he was going to the mosque. Then I told him what Abu Muhammad had said, so Ubadah (may Allah be pleased with him) said: Abu Muhammad has spoken incorrectly. I heard the Messenger of Allah (peace and blessings be upon him) say that Allah has made five prayers obligatory upon His servants. So whoever brings them (on the Day of Judgment), not having neglected any of their rights out of disdain, then Allah has promised to admit him to Paradise. And whoever does not bring them, having neglected their rights out of disdain, then Allah has no promise with him. If He wills, He will punish him, and if He wills, He will admit him to Paradise.

It is narrated from Sa'id bin Yasar (may Allah have mercy on him) that I was traveling with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) on the road to Makkah. When I feared the coming of morning, I dismounted and performed Witr, then joined him. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) asked me: Where were you? I said: I feared the coming of morning, so I dismounted and performed Witr. He said: Is there not for you the best example in the life of the Messenger of Allah (peace and blessings be upon him)? I said: Why not, by Allah! Certainly there is. He said: Then the Messenger of Allah (peace and blessings be upon him) used to perform Witr on his camel.