Sunan An-Nasai, Chapter: The Book of Purification

that the Prophet (saw) said: "I am like a father teaching you. When any one of you goes to Al-Khala (the toilet), let him not face toward the Qiblah nor turn his back toward it, and let him not clean himself with his right hand." And he used to tell them to use three stones, and he forbade using dung or old bones. (Hasan)