Musnad Ahmad 26:687

Yahya narrated to us, from Shu'bah, and Muhammad ibn Ja'far, Shu'bah narrated to us, al-Hakam narrated to us, from Ibrahim, from 'Alqamah, from 'Abdullah: That the Prophet, peace and blessings of Allah be upon him, "prayed Dhuhr as five [rak'ahs]." It was said to him, "Has something been added to the prayer?" He said, "And what is that?" They said, "You prayed five." He said, "So he folded his leg, then he prostrated two prostrations after he had finished the taslim."