Sahih Bukhari Volume 8, Book 73, Hadith Number 132.

Narated By ‘Abdullah bin ‘Umar : While the Prophet was praying, he saw sputum (on the wall) of the mosque, in the direction of the Qibla, and so he scraped it off with his hand, and the sign of disgust (was apparent from his face) and then said, “Whenever anyone of you is in prayer, he should not spit in front of him (in prayer) because Allah is in front of him.”