It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Bring me three stones." I brought him (peace and blessings be upon him) two stones and a piece of donkey dung, so he (peace and blessings be upon him) took the two stones, threw away the dung and said: "It is impure."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9924