Musnad Abu Ya'la al-Mawsili 3:362

Zuhayr narrated to us, Waki’ narrated to us, Sufyan narrated to us, from Abu Ishaq, from ‘Asim ibn Damrah, who said: We asked ‘Ali about the voluntary prayers of the Prophet, may Allah bless him and grant him peace, during the day. He said: "You will not be able to endure them." We said: "Inform us of them, so that we may take from them what we can endure." He said: "When the Prophet, may Allah bless him and grant him peace, prayed the dawn prayer, he would wait until the sun was here—meaning from the direction of the east—at the same distance as it is at the time of the afternoon prayer from here—meaning from the direction of the west—then he would stand and pray two rak'ahs. Then he would wait until the sun was here—meaning from the direction of the east—at the same distance as it is at the time of the noon prayer from here—meaning from the direction of the west—then he would stand and pray four [rak'ahs], and four before the noon prayer when the sun had passed its zenith, and two after it, and four before the afternoon prayer, separating every two rak'ahs with the taslim upon the angels brought near, the prophets, and those with them from the believers and the Muslims." ‘Ali said: "Those are sixteen rak'ahs, the voluntary prayers of the Messenger of Allah, may Allah bless him and grant him peace, during the day, and few are those who maintain them."