Musnad Ahmad 5:805

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Ishaq, who said: I heard 'Asim ibn Damrah saying: We asked 'Ali, may Allah be pleased with him, about the prayer of the Messenger of Allah, may Allah bless him and grant him peace, during the day. He said: "You would not be able to endure that." We said: "Who among us can endure that?" He said: "When the sun is here, just as it is here at the time of 'Asr, he would pray two rak'ahs. And when the sun is here, just as it is here at the time of Dhuhr, he would pray four. He would pray four before Dhuhr and two after it, and four before 'Asr. He would separate every two rak'ahs with the taslim upon the angels brought near, the prophets, and those who followed them from among the believers and the Muslims."