Ambrose of Milan
Catena Aurea · Lukasevangelium 2
And there were in the same country shepherds abiding in the field, keeping watch over their flock by night. And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid. And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people. For unto you is born this day in the city of David a Saviour, which is Christ the Lord. And this shall be a sign unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger.
Observe with what care God builds up our faith. An Angel teaches Mary; an Angel teaches Joseph; an Angel the shepherds also, of whom it is said, And there were in the same country shepherds abiding in the field.