The story of the Transfiguration appears in the synoptic gospels; Jesus took Peter, John and James up on the mountain with him to pray, and they saw him in glory with Moses and Elijah.  In ‘The Tree of Life’ by Bonaventure we hear:

“[Jesus] revealed to them the mystery of the Trinity and foretold that he would be rejected in his passion.  He showed the glory of his future resurrection in his transfiguration. The Law and the Prophets gave testimony to him in the apparition of Moses and Elijah, The Father and the Holy Spirit in the voice and the cloud.”  

Icon of the Transfiguration by Sr Theresa Margaret CHN (used with permission)