Three of the four Gospels report an event called “the transfiguration,” and we would all benefit from thinking more about that mountain scene. Patrick Schreiner has written a book (which will be out soon) called The Transfiguration of Christ: An Exegetical and Theological Reading. You should get this book!
All three accounts of the transfiguration tell us that Moses and Elijah appeared to Jesus while Jesus was on a mountain with three disciples (Peter, James, and John). We learn from Matthew 17, Mark 9, and Luke 9 that Moses and Elijah spoke with Jesus. But only in Luke 9 do we get a clue about the nature of their conversation.