Musnad Ahmad 27:1805

Muhammad ibn Abd al-Rahman narrated to us, Ayyub narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) saw phlegm in the direction of the Qibla of the mosque, so he scraped it off with his hand. Then he turned to the people, and he was angry with them, then he said: "Indeed, Allah the Exalted is in front of the face of one of you during his prayer, so let not one of you spit in front of his face during his prayer."