Musnad Ahmad 789:27

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Al-Azraq ibn Qays narrated to me, saying: I saw an old man in Al-Ahwaz praying the Asr prayer while the bridle of his mount was in his hand. It began to pull back, and he began to step back with it, while a man sitting from the Khawarij was insulting him. When he finished praying, he said: "I have heard your statement. I went on military expeditions with the Messenger of Allah (peace and blessings of Allah be upon him) six or seven times, and I witnessed his command and his ease. I would return with my mount, for it is more beloved to me than to leave it, so it would come to its familiar place, and it would be difficult for me." He (the narrator) said: I said: "How many (rak'ahs) did he pray?" He said: "Two rak'ahs." He said: And he was Abu Barzah.