On the Mount of Transfiguration, God declared, "This is my beloved son." This was not the first time God declared this, He also declared it at Jesus' baptism. But even before that, Peter writes that God spoke these words through David almost a thousand years earlier in Psalm 2.
Peter, who physically heard God repeat what He had spoken so long ago writes: "For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit."