Musnad Ahmad 32:775

Hajjaj narrated to us, Laith narrated to us, he said: Bukayr narrated to us, from Muhammad ibn Abdullah ibn Abi Sulayman, from Anas ibn Malik, that he said: "I prayed with the Messenger of Allah, may Allah bless him and grant him peace, in Mina two rak'ahs, and with Abu Bakr two rak'ahs, and with Umar two rak'ahs, and with Uthman two rak'ahs during the beginning of his caliphate."