Musnad Abu Ya'la al-Mawsili 42:20

Zuhayr narrated to us, Yahya narrated to us, from Ibn ‘Ajlan, ‘Iyad ibn ‘Abd Allah narrated to us, from Abu Sa’id al-Khudri, from the Prophet (peace be upon him): That he used to like palm-stalks, holding them in his hand. One day, he entered the mosque while he had one of them in his hand, and he saw some phlegm in the direction of the qibla of the mosque. He scraped it off with it until he had cleared it. Then he turned to the people, angry, and said: "Would any of you like for a man to face him and spit in his face? Indeed, when one of you stands for prayer, he is facing his Lord, and the angel is to his right. So let him not spit in front of him, nor to his right, but to his left or under his left foot." "If he is overcome by an urgent need, let him spit like this." And Yahya spat into his garment and folded one part of it over the other.