Musannaf Ibn Abi Shaybah

Hazrat Abdullah narrates that the Holy Prophet (peace and blessings be upon him) went out to relieve himself and said: "Find three stones for me." I brought two stones and a piece of dung to the Prophet (peace and blessings be upon him). He took the stones and threw away the dung, and said: "This is impure." And (Imam) Abu Hanifa's opinion has been mentioned that: If, after using three stones, impurity equal to the size of a dirham remains, then it will not suffice without using water.