Sunan Abi Dawud Book 2, Hadith 91
A man led the people in prayer. He spat towards qiblah while the Messenger of Allah (ﷺ) was looking at him. The Messenger of Allah said to the people when he finished his prayer: He should not lead you in prayer (henceforth).
Thenceforth he intended to lead them in prayer, but they forbade him and informed him of the prohibition of the Messenger of Allah (ﷺ). He mentioned it to the Messenger of Allah (ﷺ) who said to him: Yes.
The narrator said: I think he (the Prophet) said: You did harm to Allah and His Apostle.
The Prophet(ﷺ) liked the twigs of the date-palm, and he often had one of them in his hand. He entered the mosque and saw phlegm in the wall towards qiblah and he scraped it. He then turned towards people in anger and said: Is any one of you is pleased to spit in his face? When any of you faces qiblah, he indeed faces his Lord, the Majestic the Glorious: the angels are at right side. Therefore, he should not spit on his right side or before him towards qiblah. He should spit towards his left side or beneath his foot. If he is in a hurry, he should do so-and-so. Describing it Ibn ‘Ajlan said: He should spit in his cloth and fold a part of it over the other.
I came to the Messenger of Allah(ﷺ) who was saying prayer. He spat beneath his left foot.