Musannaf Ibn Abi Shayba 3:4553

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from Ash'ath, from al-Hakam, Hammad, and Jahm, from Ibrahim, regarding the one who is in doubt about his Witr: He said: He should make it even with a Rak'ah, and then restart the Witr.