مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4030
Musnad of Abdullah bin Mas'ood
It was narrated from `Abdullah that The Messenger of Allah (ﷺ) went out to relieve himself and said: "Bring me three stones.” I looked and I found two stones but I did not find a third, so I brought him two stones and a piece of dung, he took the two stories and threw the piece of dung away, and he said. “It is impure."
Musnad Ahmad 4056lts isnad is saheeh]