Musnad Ahmad 26:138

Waki’ narrated to us, Isra’il narrated to us, from Abu Ishaq, from Abu ‘Ubaydah, from ‘Abdullah, who said: The Prophet, peace and blessings of Allah be upon him, went out for his need, and he said to me: "Seek three stones for me." He said: So I brought him two stones and a piece of dung. He said: So he took the two stones and threw away the dung, and said: "It is filth."