Ambrose of Milan
Catena Aurea · Lukasevangelium 9
And it came to pass, as he was alone praying, his disciples were with him: and he asked them, saying, Whom say the people that I am? They answering said, John the Baptist; but some say, Elias; and others say, that one of the old prophets is risen again. He said unto them, But whom say ye that I am? Peter answering said, The Christ of God. And he straitly charged them, and commanded them to tell no man that thing; Saying, The Son of man must suffer many things, and be rejected of the elders and Chief Priests and Scribes, and be slain, and be raised the third day.
Perhaps because the Lord knew that the disciples would believe even the difficult mystery of the Passion and Resurrection, He wished to be Himself the proclaimer of His own Passion and Resurrection.