Musnad Ahmad 5:89

Waki’ narrated to us, Sufyan and Isra’il and my father narrated to us, from Abu Ishaq, from ‘Asim ibn Damrah, who said: We asked ‘Ali about the voluntary prayers of the Prophet, peace and blessings of Allah be upon him, during the day. He said: "You will not be able to endure them." We said: "Inform us of them, and we will take from them what we can endure." He said: "When the Prophet, peace and blessings of Allah be upon him, prayed the dawn prayer, he would wait until the sun was here—meaning from the east—at the same position as it is at the time of the afternoon prayer here—meaning from the west—then he would stand and pray two rak'ahs. Then he would wait until the sun was here—meaning from the east—at the same position as it is at the time of the noon prayer here—meaning from the west—then he would stand and pray four. And four before the noon prayer when the sun 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 who followed them among the believers and the Muslims." ‘Ali said: "Those are sixteen rak'ahs, the voluntary prayers of the Prophet, peace and blessings of Allah be upon him, during the day, and few are those who maintain them." Waki’ narrated to us, from his father, who said: Habib ibn Abi Thabit said to Abu Ishaq when he narrated this to him: "O Abu Ishaq, this hadith of yours is worth the fill of your mosque in gold."