Gregory the Great — as excerpted in the Catena Aurea on Luke 10:3-4

“(ubi sup.) Now every one who salutes on the way does so from the accident of the journey, not for the sake of wishing health. He then who not from love of a heavenly country, but from seeking reward, preaches salvation to his hearers, does as it were salute on the journey, since accidentally, not from any fixed intention, he desires the salvation of his hearers.”