Musnad Abu Ya'la al-Mawsili 152:211

Abu Khaythama narrated to us, Abd al-Rahman narrated to us, Shu'bah narrated to us, from al-Hakam, from Ibrahim, from Alqama, 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?" So he performed two prostrations after he had finished [the prayer] with the taslim.