Musnad Ahmad 5:674

'Abdullah narrated to us, Abu 'Abd al-Rahman ibn 'Umar narrated to us, 'Abd al-Rahim—meaning al-Razi—narrated to us, from al-'Ala' ibn al-Musayyab, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray sixteen rak'ahs at night, besides the obligatory ones."