Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1123
Anas b. Malik reported: The Messenger of Allah (may peace be upon him) said: When any one of you is engaged in prayer, he is holding intimate conversation with his Lord, so none of you must spit in front of him, or towards his right side, but towards his left side under his foot.