Musnad Ahmad 31:78

Yahya ibn Sa'id narrated to us, Ibn 'Ajlan narrated to us, 'Iyad ibn 'Abdullah narrated to me, from Abu Sa'id, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to love palm stalks; he would hold them in his hand. He entered the mosque and saw phlegm in the qibla of the mosque, so he scraped it with it until he cleaned it."