Mukhtasar Bukhari

Narrated Abdullah bin Mas'ud (may Allah be pleased with him): The Prophet (peace and blessings of Allah be upon him) went to relieve himself and ordered me to bring him three stones. I found two stones and searched for a third but could not find it, so I brought a piece of dried dung and brought it to him. The Prophet (peace and blessings of Allah be upon him) took the two stones and threw away the dung, and said: "This is impure."