Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 504
Salman reported that it was said to him: Your Apostle (may peace be upon him) teaches you about everything, even about excrement. He replied: Yes, he has forbidden us to face the Qibla at the time of excretion or urination, or cleansing with right hand or with less than three pebbles, or with dung or bone.