Musnad Ahmad 1077:11

Abu al-Yaman narrated to us, Shu'ayb narrated to us, from al-Zuhri, 'Abdullah ibn Tha'labah ibn Su'ayr al-'Udhri narrated to me, he said: And the Messenger of Allah, may Allah bless him and grant him peace, had wiped his face at the time of the Conquest. He said: That he saw Sa'd ibn Abi Waqqas—and Sa'd had witnessed Badr with the Messenger of Allah, may Allah bless him and grant him peace—performing the witr with a single rak'ah after the night prayer, meaning the 'atamah, not adding to it until he would stand in the middle of the night.