Ambrose of Milan — as excerpted in the Catena Aurea on Luke 9:32-36

“Now observe, that the cloud was not black from the darkness of condensed air, and such as to overcast the sky with a horrible gloom, but a shining cloud, from which we were not moistened with rain, but as the voice of Almighty God came forth the dew of faith was shed upon the hearts of men. For it follows, And there came a voice out of the cloud, saying, This is my beloved Son: hear ye him. Elias was not His Son. Moses was not. But this is the Son whom you see alone.”