Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) used to perform voluntary prayers at night two rak'ahs at a time. When dawn was about to break, he (peace and blessings be upon him) would perform one rak'ah of witr, and he would say: "Indeed, Allah Almighty is One, and He loves the one."
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat; its chain contains a narrator, Abdullah bin al-Walid al-Wassafi, who is da'if (weak).