Beloved brothers, our Lord and Savior sometimes gives us instruction by words and sometimes by actions. His very deeds are our commands; and whenever he acts silently he is teaching us what we should do. For example, he sends his disciples out to preach two by two, because the precept of charity is twofold - love of God and of one's neighbor.
The Lord sends his disciples out to preach in twos in order to teach us silently that whoever fails in charity toward his neighbor should by no means take upon himself the office of preaching. Rightly is it said that he sent them ahead of him into every city and place where he himself was to go. For the Lord follows after the preachers, because preaching goes ahead to prepare the way, and then when the words of exhortation have gone ahead and established truth in our minds, the Lord comes to live within us. To those who preach Isaiah says: Prepare the way of the Lord, make straight the paths of our God. And the psalmist tells them: Make a way for him who rises above the sunset. The Lord rises above the sunset because from that very place where he slept in death, he rose again and manifested a greater glory. He rises above the sunset because in his resurrection he trampled underfoot the death which he endured. Therefore, we make a way for him who rises above the sunset when we preach his glory to you, so that when he himself follows after us, he may illumine you with his love.
Let us listen now to his words as he sends his preachers forth: The harvest is great but the laborers are few. Pray therefore the Lord of the harvest to send laborers into his harvest. That the harvest is good but the laborers are few cannot be said without a heavy heart, for although there are many to hear the good news there are only a few to preach it. Indeed, see how full the world is of priests, but yet in God's harvest a true laborer is rarely to be found; although we have accepted the priestly office we do not fulfill its demands.
Think over, my beloved brothers, think over his words: Pray the Lord of the harvest to send laborers into his harvest. Pray for us so that we may be able to labor worthily on your behalf, that our tongue may not grow weary of exhortation, that after we have taken up the office of preaching our silence may not bring us condemnation from the just judge.
From here. I ran into it this week in John Leinenweber's 1990 collection of readings from Gregory, Be Friends of God.