From Sahih Bukhari (Hadith 531):
Narrated Anas:The Prophet (ﷺ) said, "Whenever anyone of you offers his prayer he is speaking in private to his Lord. So he should not spit to his right but under his left foot." Qatada said, "He should not spit in front of him but to his left or under his feet." And Shu`ba said, "He should not spit in front of him, nor to his right but to his left or under his foot." Anas said: The Prophet (ﷺ) said, "He should neither spit in the direction of his Qibla nor to his right but to his left or under his foot
Reference: Bukhari 531