Musnad Abu Ya'la al-Mawsili 154:150

Abu Khaythama narrated to us, Jarir narrated to us, from Yazid ibn Abi Ziyad, from Dawud ibn Abi ‘Asim, who said: I said to ‘Abdullah ibn ‘Umar while he was in Mina, "How much do you pray here?" He said, "The Messenger of Allah (may Allah bless him and grant him peace) prayed two rak'ahs, Abu Bakr prayed two rak'ahs, and ‘Umar prayed two rak'ahs. ‘Uthman prayed them for six years as two rak'ahs, then they prayed them as four. So, when we prayed with them, we prayed four, and when we prayed separately, we prayed two."