Musnad Ahmad 32:1016

Muhammad ibn Abdullah ibn al-Muthanna narrated to us, Humayd narrated to us, from Anas, who said: He saw phlegm in the direction of the Qibla of the mosque, and it distressed him until we recognized that on his face. So he scraped it off and said: "Indeed, when one of you, or a person, stands for prayer, he is conversing with his Lord—or his Lord is—between him and the Qibla. So if he must spit, let him spit to his left or under his foot." And he gestured like this, as if it were in his garment. He said: And we used to say to Humayd, and he would say: "Subhan Allah, who is it?" meaning the Prophet, peace and blessings of Allah be upon him, and he would not add anything more to it for us.