Musnad Ahmad 27:393

Muhammad narrated to us, Ubaydullah narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) saw phlegm in the direction of the Qibla of the mosque, so he scraped it off. Then he turned to the people and said: "When one of you is in prayer, let him not spit in front of his face, for Allah the Exalted is in front of the face of one of you when he is in prayer."