Musnad Ahmad 30:291

Yahya ibn Sa'id narrated to us, Muhammad ibn 'Ajlan narrated to us, al-Qa'qa' ibn Hakim narrated to me, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I am to you only like a father; I teach you. So when one of you comes to the privy, do not face it [the Qiblah] and do not turn your back to it, and do not clean yourselves with your right hand." And he used to command the use of three stones, and he would forbid [the use of] dung and bones.