Musnad Ahmad 303:16

'Ali ibn 'Asim narrated to us, al-Jurayri informed me, from Abu al-'Ala' ibn al-Shikhkhir, from his father, who said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, praying in his sandals." He said: "Then he cleared his throat and spat under his left sandal." He said: "Then I saw him rub it with his sandals."