Musnad Ahmad 595:9

Hassan bin Musa and Ibn Dawud narrated to us, they said: Ibn Lahi'ah narrated to us, he said: Yazid bin 'Amr and Yahya bin Ishaq narrated to us, they said: Ibn Lahi'ah informed us, from Yazid bin 'Amr al-Ma'afiri, from Abu 'Abd al-Rahman al-Hubuli, from al-Mustawrid bin 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."