Volume 1, Book 8, Number 400:
Narrated ‘Abdullah bin ‘Umar: Allah’s Apostle saw sputum on the wall of the mosque in the direc –
tion of the Qibla and scraped it off. He faced the people and said, “Whenever any one of you is pray –
ing, he should not spit in front of him because in the prayer Allah is in front of him.”