Musnad Ahmad 30:1889

Bahz narrated to us, he said: Shu'bah narrated to us, from Sa'd ibn Ibrahim, that he heard Abu Salamah narrating from Abu Hurairah, "That the Messenger of Allah (peace and blessings of Allah be upon him) prayed Dhuhr as two rak'ahs, then he performed the taslim." They said: "Has the prayer been shortened?" He said: "So he stood up and prayed two rak'ahs, then he performed the taslim, then he prostrated two prostrations after he had performed the taslim."