Musnad Ahmad 5:675

'Abdullah narrated to us, 'Abd al-Rahman 'Abdullah ibn 'Umar narrated to me, 'Abd al-Rahim al-Razi informed us, from Zakariya ibn Abi Za'idah and al-'Ala' ibn al-Musayyab, from Abu Ishaq, from 'Asim ibn Damrah, who said: We came to 'Ali ibn Abi Talib and said: "O Commander of the Faithful, will you not tell us about the voluntary prayer of the Prophet, may Allah bless him and grant him peace?" He said: "And which of you can endure it?" They said: "We will take from it what we can endure." He said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray sixteen rak'ahs during the day, besides the obligatory ones."