Musnad Ahmad 27:1846

Ya'la and Muhammad, the sons of 'Ubayd, said: Muhammad—meaning Ibn Ishaq—narrated to us, he said: Muhammad said in his narration: Nafi' narrated to me, from Ibn 'Umar, he said: I saw the Messenger of Allah, peace and blessings of Allah be upon him, with a pebble in his hand, scraping with it some phlegm he saw in the direction of the Qibla. And he was saying: "When one of you prays, let him not spit in front of him, for when the servant prays, he is standing and conversing with his Lord, the Exalted." Muhammad said: "In front of him" (wajahahu).