Musnad Ahmad 31:405

Ishaq narrated to us, Abd al-Rahman—meaning Ibn Zayd—narrated to us, from his father, from Ata ibn Yasar, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever forgets the Witr, or sleeps through it, let him pray it when he remembers it, or when he wakes up in the morning."