Musnad Ahmad 789:8

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from al-Azraq ibn Qays, who said: Abu Barzah was in al-Ahwaz on the bank of a river, and he had placed the bridle in his hand, and he began to pray. The beast began to pull back, and he began to move backward with it. A man from the Khawarij began to say: "O Allah, disgrace this old man; how is he praying?" He said: When he finished praying, he said: "I have heard your statement. I campaigned with the Messenger of Allah (peace and blessings of Allah be upon him) six, seven, or eight times, and I witnessed his affairs and his ease. My returning with my beast is easier for me than leaving it, for it pulls toward its stable, and that would be difficult for me." And Abu Barzah prayed the Asr prayer as two rak'ahs.