Musnad Ahmad 31:274

Waki' narrated to us, Abd al-Rahman ibn Zayd ibn Aslam narrated to us, from his father, from Ata ibn Yasar, from Abu Sa'id al-Khudri, who said: The Prophet, peace and blessings of Allah be upon him, said: "Whoever sleeps through the Witr or forgets it, let him perform Witr when he remembers it or wakes up."