Musannaf Ibn Abi Shayba 3:5072

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Muhammad ibn ‘Ajlan, from ‘Iyad ibn ‘Abd Allah ibn Sa‘d ibn Abi Sarh, from Abu Sa‘id, who said: The Messenger of Allah (may Allah bless him and grant him peace) entered the mosque and in his hand was a date-palm stalk, and he used to love date-palm stalks. He saw phlegm in the direction of the Qibla, so he scraped it off. Then he turned to the people and said: "O people, when one of you stands to pray, Allah the Almighty and Majestic faces him, and on his right is an angel. Would any of you like for a man to face him and spit in his face? So let none of you spit in the direction of the Qibla, nor to his right. Let him spit under his left foot or to his left. If he is overcome by an urgent need, let him spit like this"—meaning into his garment.