Musannaf Ibn Abi Shayba 1:1259

Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from Salim ibn Abi al-Dhayyal, from al-Hasan, regarding a man who prayed, and when he finished his prayer, he saw chicken excrement on his garment. He said: "It is only a bird."