Gregory the Great — as excerpted in the Catena Aurea on Matthew 11:1-6

“(Hom. in Ev. vi. 1.) We must enquire how John, who is a prophet and more than a prophet, who made known the Lord when He came to be baptized, saying, Behold the Lamb of God, that taketh away the sins of the world!—why, when he was afterwards cast into prison, he should send his disciples to ask, Art thou he that should come, or look we for another? Did he not know Him whom he had pointed out to others; or was he uncertain whether this was He, whom by foretelling, by baptizing, and by making known, he had proclaimed to be He?”