Matthew 17:9 - Ellicott's Commentary On The Whole Bible

Tell the vision to no man. — The command obviously included even the rest of the Apostles within the range of its prohibition. For them in their lower stage of spiritual growth, the report of the vision at second hand would either have led them to distrust it or to pervert its meaning. Whatever reasons excluded them from being spectators were of still greater weight for the time against their hearing of what had been seen from others. The Greek word for “vision,” it may be noted, means simply “what they had seen,” and does not suggest, as the English word does, the thought of a dream-state in the beholders.