Abd Khayr narrates: We were in the mosque when, in the last part of the night, Sayyiduna Ali (may Allah be pleased with him) came out to us and said: Where is the person who was asking about the witr? We gathered around him, and he said that the Prophet (peace and blessings be upon him) would perform the witr in the early part of the night, and also in its middle part, and also at this hour (that is, in the last part of the night). When he (peace and blessings be upon him) passed away, he was performing the witr at this hour.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat; its chain contains a narrator, Abu Shaybah, who is da'if (weak).