Sayyiduna Kharijah bin Hudhafah al-Adawi (may Allah be pleased with him) said: I heard the Prophet (peace and blessings be upon him) say: "Allah Almighty has aided you with a prayer that is better for you than red camels; its time is between Isha and the rising of dawn, that is, witr."
Imam al-Bukhari (may Allah have mercy on him) said: It is not known that the narrators of its chain heard from one another.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4511
Abu Nadrah (may Allah have mercy on him) said that Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) informed him that they asked the Prophet (peace and blessings be upon him) about witr, and he (peace and blessings be upon him) said: "The time of witr is before the morning prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4512
It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Pray witr before morning comes."
(b) Through another chain, it is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever reaches the time of morning and has not prayed witr, there is no witr for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4513
Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) regarding making up witr; this will come later.
The narration of Yahya bin Abi Kathir appears to be the more sound.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4514
Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Hasten to pray witr before morning."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4515
Sayyiduna Ibn Umar (may Allah be pleased with them both) said: Whoever prays at night, let him make witr his last prayer, for the Prophet (peace and blessings be upon him) commanded that. When dawn breaks, the time of the night prayer and of witr is over, because he (peace and blessings be upon him) said: "Witr is before dawn."
(b) In a narration of al-Fahham, he (peace and blessings be upon him) said: "Pray witr at dawn."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4516