Musnad Ahmad 5:629

'Abd al-Razzaq narrated to us, Isra'il narrated to us, from 'Abd al-A'la, from Muhammad ibn 'Ali, from 'Ali: "That the Prophet, may Allah bless him and grant him peace, used to fast continuously (wisal) from the time before dawn (al-sahar) to the time before dawn (al-sahar)."