Musnad Ahmad 31:197

Yahya narrated to us, from Ibn ‘Ajlan, who said: ‘Iyad ibn ‘Abdullah narrated to me, from Abu Sa’id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) used to like palm-stalks, and he would hold one in his hand. One day, he entered the mosque while holding one of them in his hand, and he saw some phlegm in the direction of the qiblah of the mosque. He scraped it off with it until he had cleaned it. Then he turned to the people, angry, and said: "Would any of you like for a man to face him and spit in his face? When one of you stands for prayer, he is facing his Lord, the Almighty and Majestic, and the angel is to his right. So let him not spit in front of him, nor to his right. Let him spit under his left foot or to his left. If he is overcome by an urgent need, let him do like this." And he rubbed one part of it against the other. And Yahya spat into his garment and rubbed it.