Abu Dawud Book 1, Hadith Number 7.
Chapter : Disapproval of facing the qibla while releiving oneself.
Narated By Salman al-Farsi : It was said to Salman: Your Prophet teaches you everything, even about excrement. He replied: Yes. He has forbidden us to face the qiblah at the time of easing or urinating, and cleansing with right hand, and cleansing with less than three stones, or cleansing with dung or bone.