Musnad Ahmad 26:884

Husayn ibn Muhammad 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, and I was with him. He said to me: "Seek three stones for me." He said: So I found two stones and a piece of dung for him. He said: I brought them to him, so he took the two stones and threw away the dung. And he said: "This is filth."