Musnad Abu Ya'la al-Mawsili 152:268

Abu Khaythama narrated to us, al-Hasan ibn Musa narrated to us, Zuhayr narrated to us, Abu Ishaq narrated to us, he said: It is not Abu Ubaydah who mentioned it, but rather Abd al-Rahman ibn al-Aswad—I think—from Abdullah, who said: The Prophet, peace and blessings of Allah be upon him, came to relieve himself and ordered me to bring him three stones. I took two stones and searched for the third but did not find it. So I took a piece of dung and brought them to the Prophet, peace and blessings of Allah be upon him. He took the two stones and threw away the dung, and said: "This is filth (rijs)."