Mark 9:2-4

2Six days later, Jesus took with Him Peter and James and John, and led them up on a high mountain by themselves. And He was transfigured (changed in form) before them [and began to shine brightly with divine and regal glory];

3and His clothes became radiant and dazzling, intensely white, as no launderer on earth can whiten them.

4Elijah appeared to them along with Moses, and they were having a conversation with Jesus.