Ambrose of Milan
Catena Aurea · Lukasevangelium 22
Then came the day of unleavened bread, when the Passover must be killed. And he sent Peter and John, saying, Go and prepare us the Passover, that we may eat. And they said unto him, Where wilt thou that we prepare? And he said unto them, Behold, when ye are entered into the city, there shall a man meet you, bearing a pitcher of water; follow him into the house where he entereth in. And ye shall say unto the goodman of the house, The Master saith unto thee, Where is the guestchamber, where I shall eat the Passover with my disciples? And he shall shew you a large upper room furnished: there make ready. And they went, and found as he had said unto them: and they made ready the Passover.
Or the pitcher is a more perfect measure, but the water is that which was thought meet to be a sacrament of Christ; to wash, not to be washed.