Musnad Abu Ya'la al-Mawsili 42:316

Abu Khaythama narrated to us, 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 Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever sleeps through the Witr or forgets it, let him pray it when he remembers or wakes up."