Musnad Abu Ya'la al-Mawsili 154:91

Daud ibn 'Amr al-Dabbi narrated to us, Salih ibn 'Umar narrated to us, from Yazid ibn Abi Ziyad, from Daud ibn 'Asim ibn 'Urwah ibn Mas'ud al-Thaqafi, who said: I asked Ibn 'Umar in Mina about the prayer here. He said: I prayed with the Messenger of Allah (may Allah bless him and grant him peace) two rak'ahs, and with Abu Bakr two rak'ahs, and with 'Umar two rak'ahs, and with 'Uthman for a portion of his caliphate two rak'ahs. When we prayed with them, we prayed as they prayed, and when we prayed alone, we prayed [four rak'ahs].