Mark 9:4

And there appeared unto them Elias with Moses
Or Moses and Elias, as all the Oriental versions read, as in Mt. 17:3, (See Gill on Matthew 17:3):

and they were talking with Jesus;
concerning his decease, and what he was to do and suffer at Jerusalem, and of which he himself had lately talked with his disciples; so that this might have been a confirmation of these things to them; (See Gill on Matthew 17:3).