Musnad Ahmad 5:138

Hujayn ibn al-Muthanna narrated to us, Isra'il narrated to us, from 'Abd al-A'la, from Abu 'Abd al-Rahman, from 'Ali, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to continue fasting (without breaking it) until the time of the pre-dawn meal (al-sahar)."