Musnad Ahmad 303:7

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Sa'id al-Jurayri, from Abu al-'Ala' ibn 'Abd Allah ibn al-Shikhkhir, from his father, who said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, praying, then he spat under his foot, then he rubbed it with his sandal while it was on his foot."