It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) went out to relieve himself and said to me, "Bring me three stones." I was able to find two stones and a dry piece of dung. The Prophet (peace and blessings be upon him) took the two stones and threw away the piece of dung, saying: "This is impure."