Musnad Ahmad 27:736

Yahya narrated to us, from Isa ibn Hafs, my father narrated to me, saying: I was with Ibn Umar on a journey, and he prayed the Dhuhr and Asr [prayers] as two rak'ahs each. Then he stood up to a rug and saw people performing voluntary prayers (tasbih) after them. He said: "What are these people doing?" I said: "They are performing voluntary prayers." He said: "If I were to pray before them or after them, I would have completed them [as four rak'ahs]. I accompanied the Prophet (peace and blessings of Allah be upon him) until he passed away, and he would not add to two rak'ahs. And [I accompanied] Abu Bakr until he passed away, and he would not add to them, and Umar and Uthman likewise."