Musnad Ahmad 26:749

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Abu Ishaq, from 'Alqamah ibn Qays, from Ibn Mas'ud: That the Prophet, peace and blessings of Allah be upon him, went to relieve himself, and he ordered Ibn Mas'ud to bring him three stones. He brought him two stones and a piece of dung. He threw away the dung and said, "It is filth; bring me a stone."