Musnad Ahmad 27:729

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, from Nafi', 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]."