Musnad Abu Ya'la al-Mawsili 3:245

Ubayd Allah ibn Umar narrated to us, Mansur ibn Abd Allah al-Thaqafi narrated to us, Muhammad ibn Umar ibn Ali ibn Abi Talib narrated to us, from his father, from Ali ibn Abi Talib, who said: The slogan of the Prophet, may Allah bless him and grant him peace, was: "Eat the best."