Is this strange narration authentic?!
One day ‘Ali attacked Amr ibn Aas, he threw a spear and Amr fell to the ground, Amr fell to the ground and he then exposed his buttocks. ‘Ali then turned away his face [people said] this was Amr bin Aas. ‘Ali replied he showed me his anus and this made me merciful to him. When Amr ibn Aas returned, Mu’awiya said ‘You should praise Allah and your anus”.
Al Bidayah wa al Nihayah, Volume 7 page 293