From Sahih Bukhari (Hadith 1213):
Narrated Ibn `Umar:The Prophet (ﷺ) saw some sputum on the wall facing the Qibla of the mosque and became furious with the people of the mosque and said, "During the prayer, Allah is in front of everyone of you and so he should not spit (or said, 'He should not expectorate')." Then he got down and scratched the sputum with his hand. Ibn `Umar said (after narrating), "If anyone of you has to spit during the prayer, he should spit to his left
Reference: Bukhari 1213