Musannaf Ibn Abi Shayba 35:263

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from Abu ‘Ubaydah, from ‘Abdullah, who said: The Prophet, may Allah bless him and grant him peace, went out for his need and said, "Seek three stones for me." I brought him two stones and a piece of dung. He took the two stones and threw away the dung, and said, "It is filth (rijs)." And he mentioned that Abu Hanifah said: It does not suffice him until he performs wudu’ if there remains after the three stones [an amount] more than the size of a dirham.