Sahih Muslim 550 a
The Book of Mosques and Places of Prayer
The prohibition of spitting in the masjid, during prayer and at other times. The prohibition of a praying person spitting in front of him or to his right
Abu Huraira reported that the Messenger of Allah (ﷺ) saw some sputum in the direction of the Qibla of the mosque. He turned towards people and said: How Is it that someone amongst you stands before his Lord and then spits out in front of Him? Does any one of you like that he should be made to stand in front of someone and then spit at his face? So when any one of you spits, he must spit on his left side under his foot. But if he does not find (space to spit) he should do like this. Qasim (one of the narrators) spat in his cloth and then folded it and rubbed it.