Musnad Abu Ya'la al-Mawsili 42:141

Narrated to us Ishaq, narrated to us Waki’ from ‘Abd al-Rahman ibn Zayd, from his father, from ‘Ata’ ibn Yasar, from Abu Sa’id, 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 wakes up or remembers it."