Musnad Ahmad 27:205

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' informed me, from Ibn 'Umar, who said: "I prayed with the Prophet, peace and blessings of Allah be upon him, at Mina two rak'ahs, and with Abu Bakr, and with 'Umar, and with 'Uthman during the beginning of his caliphate; then he completed [the prayer]."