Sahih Bukhari | Volume 1 | Hadith Number 408

Narrated by Abu Huraira
Prophet said, "If anyone of you stands for prayer, he should not spit in front of him because in prayer he is speaking in private to Allah and he should not spit on his right as there is an angel, but he can spit either on his left or under his left foot and bury it (i.e. expectoration)."