Musnad Abu Ya'la al-Mawsili 3:272

Yahya ibn Ayyub narrated to us, Muhammad ibn al-Hajjaj al-Lakhmi narrated to us, Abd al-Malik ibn Umayr narrated to us, from al-Nazzal ibn Sabra, from Ali ibn Abi Talib, from the Prophet (may Allah bless him and grant him peace), who said: "The adornment of prayer is the sandal."