Sayyiduna Sahl bin Sa'd al-Sa'idi (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) was asked about cleansing oneself after relieving oneself (istinja), and he (peace and blessings be upon him) said: "Does not one of you find three stones? Two stones for the two sides of the buttocks and one stone for the place of passage." This is how it is in his book.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 553