Musnad Abu Ya'la al-Mawsili 129:136

And from Anas bin Malik, he said: The Messenger of Allah, peace and blessings of Allah be upon him, entered the mosque and saw phlegm on its wall. The severity of that was evident upon him. He said: "Indeed, when one of you stands to pray, he is only standing to converse with his Lord, or his Lord is between him and the Qiblah. So if one of you spits, let him spit to his left, under his foot, or let him spit like this." And he spat into the edge of his garment and rubbed some of it against the other.