Musnad Ahmad 5:67

Yahya ibn Sa'id narrated to us, from Sufyan, Muhammad ibn 'Umar ibn 'Ali narrated to us, from 'Ali, who said: I said, "O Messenger of Allah, when you send me, should I be like a heated iron rod, or should I be like the witness who sees what the absent one does not see?" He said: "The witness sees what the absent one does not see."