Musnad Abu Ya'la al-Mawsili 152:309

Abu Khaythama narrated to us, Jarir narrated to us, from Mughira, from his companions, from Ibrahim, from al-Aswad, who said: I was with Abdullah at Mina. When Uthman prayed four rak'ahs there, Abdullah said when he finished his prayer: "I prayed with the Messenger of Allah (ﷺ) in this place two rak'ahs, and Abu Bakr prayed two rak'ahs, and Umar prayed two rak'ahs." He said: I think he mentioned that Uthman had prayed two rak'ahs, then he said: "He prayed four today." Al-Aswad said: I said to him, "O Abu Abd al-Rahman, why did you not perform the taslim after the two rak'ahs and pray the other two rak'ahs afterwards?" He said: "Dissension is evil."