Musannaf Ibn Abi Shayba 3:4377

Hushaym narrated to us, from Husayn, who said: Abu Zabyan narrated to us, who said: Ali used to come out to us while we were waiting for the first light of dawn, and he would say: "The prayer, the prayer! Indeed, this is the hour of the Witr." Then, when the dawn broke, he would pray two rak'ahs. Then the prayer (the obligatory dawn prayer) was called, and he would pray.