Sunan An-Nasai, Chapter: The Book of Leading the Prayer (Al-Imamah)

that Asim bin Damrah said: "I asked Ali bin Abi Talib about the prayer of the Messenger of Allah (saw) during the day before the prescribed prayers. He said: 'Who is able to do that?' Then he told us: 'The Messenger of Allah (saw) used to pray two Rakahs when the sun had passed its zenith, and four Rakahs before the middle of the day, with the Taslim at the end.'" (Hasan)