Musnad Ahmad 27:959

Abu Salamah narrated to us, Layth informed us, from Nafi’, from Ibn ‘Umar, that the Prophet (peace and blessings of Allah be upon him) saw phlegm in the direction of the Qiblah of the mosque while he was praying in front of the people. He scraped it off, then said when he finished the prayer: "Indeed, when one of you is in prayer, Allah, the Almighty and Majestic, is before his face. So let no one spit before his face while in prayer."