Musnad Ahmad 5:719

Abdullah narrated to us, Abu Bakr narrated to me, ‘Amr ibn Hammad narrated to us, from Asbat ibn Nasr, from Simak, from Hanash, from ‘Ali: That the Prophet, peace and blessings of Allah be upon him, when he sent him with [the chapter of] Bara’ah, he said: "O Prophet of Allah, I am not eloquent, nor am I a public speaker." He said: "It must be that I go with it, or you go with it." He said: "If it must be, then I will go." He said: "Then go, for Allah will make your tongue firm and guide your heart." He said: Then he placed his hand upon his mouth.