Musnad Abu Ya'la al-Mawsili 42:297

Zuhayr narrated to us, Yazid narrated to us, Fudayl ibn Marzuq informed us, from Atiyyah al-Awfi, from Abu Sa’id, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to pray the Duha (forenoon) prayer until we would say, "He will not abandon it." And he would abandon it until we would say, "He will not pray it."