Musnad Ahmad 26:19

'Amr ibn al-Haytham narrated to us, Shu'bah narrated to us, from al-Hakam, 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: "Has something been added to the prayer?" It was said: "You prayed five." "So he performed two prostrations."