Musnad Abu Ya'la al-Mawsili 3:336

Abu Khaythama narrated to us, Muhammad ibn Fudayl narrated to us, Mughira narrated to us, from Umm Musa, from Ali, who said: The last speech of the Messenger of Allah, may Allah bless him and grant him peace, was: "The prayer, the prayer. Fear Allah regarding what your right hands possess."