Musnad Ahmad 30:2911

Muhammad ibn Ja'far and Bahz narrated to us, they said: Shu'bah narrated to us, from Sa'd ibn Ibrahim, from Abu Salamah—Bahz said: That he heard Abu Salamah—from Abu Hurairah: "That the Messenger of Allah (ﷺ) 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."