Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1118
Abu Sa'id al-Khudri reported: The Apostle of Allah (may peace be upon him) saw sputum sticking to the Qibla of the mosque. He scratched it off with a pebble and then forbade spitting on the right side or in front, but (it is permissible) to spit on the left side or under the left foot.