Musnad Ahmad 595:3

Musa ibn Dawud narrated to us, he said: Ibn Lahi'ah informed us, from Yazid ibn 'Amr, from Abu 'Abd al-Rahman al-Hubuli, from al-Mustawrid ibn Shaddad, the companion of the Prophet, may Allah bless him and grant him peace, who said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, when he performed ablution, he would interlace the toes of his feet with his little finger."