Musnad Ahmad 26:506

Sulayman ibn Dawud 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, from his father, from Abdullah, that the Messenger of Allah (peace and blessings of Allah be upon him) went to relieve himself and said: "Bring me three stones." I searched and found two stones, but I did not find a third. So I brought him two stones and a piece of dung. He took the two stones and threw away the dung, and said: "It is filth (rijs)."