Musnad Ahmad 26:484

Ibn Numayr narrated to us, al-A’mash informed us, from Ibrahim, from ‘Abd al-Rahman ibn Yazid, who said: ‘Abdullah said, when he saw ‘Uthman praying four rak’ahs at Mina: "I prayed behind the Messenger of Allah (peace and blessings of Allah be upon him) two rak’ahs, and behind Abu Bakr two rak’ahs, and behind ‘Umar two rak’ahs. Would that my share from the four were two accepted rak’ahs."