Musnad Ahmad 5:667

'Abdullah narrated to us, Al-'Abbas ibn al-Walid narrated to me, Abu 'Awanah narrated to us, from Abu Ishaq, from 'Asim ibn Damrah, who said: 'Ali was asked about the prayer of the Messenger of Allah, may Allah bless him and grant him peace. He said: "He used to pray sixteen rak'ahs at night."