Sahih Bukhari
Prayers (Salat)
Volume: 001 / Book Number: 008 / Hadith Number: 406
Narrated Abu Said:
The Prophet saw sputum on (the wall of) the mosque in the direction of the Qibla and scraped it off with gravel. Then he forbade Spitting in front or on the right, but allowed it on one's left or under one's left foot.