It is narrated from Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever cleanses himself after relieving himself (istinja) with three stones in which there is no dung, they will be a means of purification for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3714