Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: I went out with the Prophet (peace and blessings be upon him) when he went to relieve himself, and he (peace and blessings be upon him) ordered me to bring three stones. I brought two stones and a piece of dung, so he (peace and blessings be upon him) took the two stones, threw away the dung and said: "This is filth."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9919