Musannaf Ibn Abi Shayba 3:4354

Waki’ narrated to us, he said: Sufyan narrated to us, from Abu Qays al-Awdi, from Abd al-Rahman ibn Tharwan, he said: I asked Amr ibn Maymun about the man who performs Witr, then wakes up. He said: He should make it even by adding one rak'ah.