Musnad Ahmad 27:1892

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Salim, from Ibn 'Umar, who said: "I prayed with the Messenger of Allah, peace and blessings of Allah be upon him, at Mina two rak'ahs; and with Abu Bakr two rak'ahs; and with 'Umar two rak'ahs; and with 'Uthman for a portion of his caliphate, then he prayed them as four."