Musnad Ahmad 32:1123

Yazid narrated to us, Humaid informed us, from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) saw phlegm in the direction of the Qibla of the mosque, so he scraped it off. It was seen on his face how severe that was for him. He said: "Indeed, when the servant stands to pray, he is only conversing with his Lord, between himself and the Qibla. So if one of you spits, let him spit to his left, or under his left foot, or do like this." And he took the edge of his garment and spat into it, then rubbed some of it against the other.