Musnad Abu Ya'la al-Mawsili 152:59

Muhammad ibn Abi Bakr narrated to us, Yahya narrated to us, from Zuhayr ibn Mu'awiyah, from Abu Ishaq, from 'Abd al-Rahman ibn al-Aswad, from his father, from 'Abdullah, who said: I was with the Prophet, may Allah bless him and grant him peace, and he came to relieve himself. He said, "Seek three stones for me." I did not find anything except two stones and a piece of dung. He took the two stones and rejected the dung, and said, "This is filth."