It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked about istinja (cleansing after relieving oneself), and he (peace and blessings be upon him) said: "Can one of you not find three stones? Two stones for the two sides, and one stone for the anal passage."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5676