It was narrated from 'Abdullah bin Mas'ud:
"The Messenger of Allah went to the toilet and said: 'Bring me three stones.' So I brought him two stones and a piece of dung. He took the two stones and threw the dung away, saying: 'It is impure.'" (ES)
English reference (ES): Vol. 1, Book 1, Hadith 314 (ES)
Arabic reference (ES): Book 1, Hadith 335 (ES)