Musannaf Ibn Abi Shayba 3:3589

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Abu Ishaq, from Asim ibn Damrah, who said: Some of the companions of Ali said to Ali: "Will you not tell us about the voluntary prayer of the Messenger of Allah (peace and blessings of Allah be upon him) during the day?" He said: Ali said: "You will not be able to endure it." They said: "Inform us of it, so that we may take from it what we can endure." He said: Ali said: "When the sun rose from its east and was in the same position as it is in the west at the time of the Asr prayer, he would pray two rak'ahs. And when it was in the east in the same position as it is in the west at the time of the Dhuhr prayer, he would pray four rak'ahs. He would pray four rak'ahs before Dhuhr and two rak'ahs after Dhuhr. And he would pray four rak'ahs before Asr, saying the taslim in every two rak'ahs to the angels brought near, the prophets, and those who followed them among the believers and the Muslims."