Musnad Ahmad 26:867

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Jabir, from 'Abd al-Rahman ibn al-Aswad, from his father, from 'Abdullah, who said: "The Prophet of Allah (peace and blessings of Allah be upon him) prayed Dhuhr as five [rak'ahs]." They said: "Has something been added to the prayer?" So he performed two prostrations.