(Saw this quoted in an essay by Bishop Wallace Benn)
It is from On Christian Doctrine, Vol 4, Chapter 15, Section 32
And so our Christian orator, while he says what is just, and holy and good (and he ought never to say anything else), does all he can to be heard with intelligence, with pleasure and with obedience; and he need and so far as he succeeds, he will succeed more by piety in prayer than by gifts of oratory; and so he ought to pray for himself and for those he is about to address, before he attempts to speak. And when the hour is come that he must speak, he ought, before he opens his mouth, to lift up his thirsty soul to God, to drink in what he is about to pour forth and to be himself filled with what he is about to distribute. For, as in regard to every matter of faith and love there are many things that may be said, and many ways of saying them, who knows what it is expedient at a given moment for us to say, or to be heard saying, except God who knows the hearts of all? And who can make us say what we ought and in the way we ought except Him in whose hand both we and our speeches are? Accordingly, he who is anxious both to know and to teach should learn all that is to be taught, and acquire such a faculty of speech as is suitable for a divine. But when the hour for speech arrives, let him reflect upon that saying of our Lord's as better suited to the wants of a pious mind "Take no thought how or what ye shall speak; for it shall be given you in that same hour what ye shall speak. For it is not ye that speak, but the Spirit of your Father which speaketh in you." The Holy Spirit, then, speaks thus in those who for Christ's sake are delivered to the persecutors; why not also in those who deliver Christ's message to those who are willing to learn?