Musnad Ahmad 5:410

Ghassan ibn al-Rabi' narrated to us, Abu Isra'il narrated to us, from al-Suddi, from 'Abd Khayr, who said: Ali ibn Abi Talib came out to us while we were in the mosque and said: "Where is the one asking about the Witr?" So, whoever among us was in a rak'ah joined another to it until we gathered around him. He said: "Indeed, the Messenger of Allah, peace and blessings of Allah be upon him, used to perform Witr at the beginning of the night, then he performed Witr in the middle of it, then he established the Witr at this hour." He said: "And that is at the break of dawn."