Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) went to relieve himself and he (peace and blessings be upon him) ordered me to bring three stones. I found two stones, and despite searching for a third I could not find one, so I picked up a piece of dung and brought them (all three) to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) took the two stones, threw away the dung and said: "Hadha riks" (This is impure).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4144