Musnad Ahmad 27:703

Yahya narrated to us, from Ubaydullah, Nafi’ informed us, 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. He scraped it off, then said: "When one of you is in prayer, he should not spit, for Allah the Exalted is in front of the face of one of you while in prayer."