Musannaf Ibn Abi Shayba 3:3982

Abu Bakr narrated to us, he said: Abu Usama narrated to us, from Ibn 'Awn, he said: I asked Ibrahim about a man who performs Witr at the end of the night while some of the night remains, so he begins [the prayer] and recites. Then, when dawn breaks, he bows for one Rak'ah, then he adds another to it, so they become the two Rak'ahs of Fajr. He said: So I mentioned that to Muhammad ibn Sirin, and he said: "I do not know what this is."