Musannaf Ibn Abi Shayba 3:5084

Ibn Numayr narrated to us, he said: ‘Abdullah bin ‘Umar narrated to us, from Nafi’, from Ibn ‘Umar, from the Prophet (peace be upon him), that he saw phlegm in the direction of the Qibla of the mosque, so he scraped it off. And he said: "When one of you stands in prayer, let him not spit in front of his face, for Allah is in front of the face of one of you when he is in prayer."