Musnad Ahmad 303:18

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: Al-Jurayri informed us, from Abu al-'Ala', from Mutarrif ibn 'Abdullah, from his father: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to pray and spit under his left foot."