Abd al-Rahman bin al-Aswad (may Allah have mercy on him) reports from his father that he heard Sayyiduna Abdullah (may Allah be pleased with him) say: The Prophet (peace and blessings be upon him) went to relieve himself and ordered me to bring him three stones. I found two stones and searched for the third but could not find it, so I took a piece of dung and brought it 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: "This is riks (filth)."
Imam Abu Isa al-Tirmidhi (may Allah have mercy on him) said: In my view the narration of Isra'il is more established and more authentic, because Isra'il is more reliable in narrating from Abu Ishaq than these others, and Qays bin al-Rabi' corroborated him in that. He said: Zuhayr is not that strong in Abu Ishaq, because he heard from Abu Ishaq late, and in the last part of his life Abu Ishaq's memory had deteriorated.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 526