Musnad Ahmad 3941

Abdur-Rahman bin al-Aswad (narrated) from his father that he heard ‘Abdullah bin Mas'ood (رضي الله عنه) say: The Prophet (ﷺ) went to relieve himself and he instructed me to bring him three stones. I found two stones and I did not find a third, so I picked up a piece of dung, and I brought them to the Prophet (ﷺ). He took the two stones and threw away the dung and said: "It is impure."