Musnad Abu Ya'la al-Mawsili 3:235

Uthman ibn Abi Shaybah narrated to us, Sa'id ibn Khuthaym narrated to us, Fudayl ibn Marzuq narrated to us, from Abu Ishaq, from Asim ibn Damrah, from Ali, who said: The Prophet, peace and blessings of Allah be upon him, used to pray eight rak'ahs of voluntary prayer at night, and twelve rak'ahs during the day.