Musnad Ahmad 5:132

Bakr ibn Isa al-Rasibi narrated to us, Umar ibn al-Fadl narrated to us, from Nu'aym ibn Yazid, from Ali ibn Abi Talib, who said: The Prophet, may Allah bless him and grant him peace, commanded me to bring him a tablet so that he might write upon it that which his nation would not go astray after him. He said: I feared that his soul might depart from me. He said: I said: "I have memorized and I retain." He said: "I enjoin prayer, and zakat, and what your right hands possess."