It is narrated from Sayyiduna Abdullah (may Allah be pleased with him): The Messenger of Allah (peace and blessings be upon him) ordered me to bring him three stones, so I brought two stones and a piece of dung. He (peace and blessings be upon him) took the stones, threw away the dung and said: "Bring me a stone."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 503