Musnad Abu Ya'la al-Mawsili 152:126

Abu Khaythama narrated to us, Muhammad ibn Khazim narrated to us, al-A'mash narrated to us, from Ibrahim, from 'Abd al-Rahman ibn Yazid, from 'Abdullah, who said: 'Uthman prayed four [rak'ahs] at Mina. 'Abdullah said to him: "I prayed with the Prophet (peace and blessings of Allah be upon him) at Mina two rak'ahs, and with Abu Bakr two rak'ahs, and with 'Umar two rak'ahs. Then the paths diverged for you. I would love to have two accepted rak'ahs instead of your four rak'ahs."