Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Make witr the last of your prayer at night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4828
Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both): "Whoever prays at night, let him make witr the last of his night prayer, before the morning." The Prophet (peace and blessings be upon him) used to command (them) in this way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4829
Abdullah bin Shaqiq (may Allah have mercy on him) narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that a man asked the Prophet (peace and blessings be upon him) while I was between the Prophet (peace and blessings be upon him) and the questioner. He said: O Messenger of Allah! How is the night prayer (to be offered)? He (peace and blessings be upon him) said: "Two by two rak'ahs; when you fear (the coming of) the morning, pray one rak'ah, and make witr your last prayer." Then, a year later, a man asked the Prophet (peace and blessings be upon him) at the same place; I do not know whether it was the same man or someone else, and he (peace and blessings be upon him) told him the same thing.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4830
Al-Aswad (may Allah have mercy on him) narrates from Sayyidah Aishah (may Allah be pleased with her) that when he (peace and blessings be upon him) prayed at night, his (peace and blessings be upon him) last prayer would be the witr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4831
Al-Aswad bin Yazid came to Aishah (may Allah be pleased with her) and asked about the night prayer of the Prophet (peace and blessings be upon him), and she said: "The night prayer of the Prophet (peace and blessings be upon him) used to be thirteen rak'ahs. Then he (peace and blessings be upon him) prayed eleven rak'ahs and left two rak'ahs. Then, until his death, he (peace and blessings be upon him) prayed nine rak'ahs at night. His (peace and blessings be upon him) last prayer of the night was the witr."
Muhammad bin Ishaq bin Khuzaymah differed from him, narrating it from Mu'ammal bin Hisham through Abu Ishaq from Masruq; but the narration of Abu Dawud is more correct, as shown by the narration of Ammar bin Ruzayq mentioned earlier.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4832