2 Peter 1:16

The apostles had not preached myths to their hearers, as the false teachers to whom Peter referred later in this epistle were doing. The apostles’ testimony rested on historical events that they had observed personally. They had seen Jesus’ power in action during His first coming as God’s anointed Messiah. Jesus Christ’s majesty appeared especially clearly on the Mount of Transfiguration (Matthew 17:1-8). "Power" and "coming" are a hendiadys that means "powerful coming" with emphasis on the fact that Jesus’ coming was with power. This is the only explicit mention of the Transfiguration outside the Synoptic Gospels.