Musnad Ahmad 5:694

'Abdullah narrated to us, 'Uthman ibn Abi Shaybah narrated to me, Sa'id ibn Khuthaym Abu Ma'mar al-Hilali 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."