Musnad Ahmad 26:416

Yahya ibn Adam narrated to us, Zuhayr narrated to us, Abu Ishaq narrated to us, he said: It was not Abu Ubaydah who mentioned it, but rather Abd al-Rahman ibn al-Aswad, from his father, that he heard Abdullah ibn Mas'ud saying: The Prophet, peace and blessings of Allah be upon him, went to relieve himself, and he ordered me to bring him three stones. I found two stones, and I did not find the third, so I took a piece of dung. I 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."