Sahih Bukhari Volume 2, Book 22, Hadith Number 302.

Narated By Al-Azraq bin Qais : We were at Al-Ahwaz fighting the AlHaruriya (tribe). While I was at the bank of a river a man was praying and the reins of his animal were in his hands and the animal was struggling and he was following the animal. (Shu’ba, a sub-narrator, said that man was Abu Barza al-Aslaml). A man from the Khawarij said, “O Allah! Be harsh to this sheik.” And when the sheik (Abu Barza) finished his prayer, he said, “I heard your remark. No doubt, I participated with Allah’s Apostle in six or seven or eight holy battles and saw his leniency, and no doubt, I would rather retain my animal than let it return to its stable, as it would cause me much trouble.”