Musannaf Ibn Abi Shayba 3:2134

Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, from Abu Salamah, from Abu Hurairah, that the Prophet (peace and blessings of Allah be upon him) prayed the Dhuhr prayer as two rak'ahs, then he performed the taslim. It was said to him, "Has the prayer been shortened?" So he prayed two other rak'ahs, then he performed the taslim, then he prostrated two prostrations.