Musnad Ahmad 32:524

Yunus ibn Muhammad narrated to us, Layth—meaning Ibn Sa'd—narrated to us, from Bukayr ibn 'Abdullah, from Muhammad ibn 'Abdullah ibn Abi Sulaym, from Anas ibn Malik, that he said: "I prayed with the Messenger of Allah (may Allah bless him and grant him peace) at 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."