Musnad Ahmad 27:62

Isma'il narrated to us, Ayyub 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 Qibla of the mosque. He stood up and scraped it—or he said: he rubbed it—with his hand. Then he turned to the people, and he was angry with them, and said: "Indeed, Allah, the Almighty and Majestic, 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."