Mark 9:9

And as they came down from the mountain
Christ and his three disciples, Peter, James, and John, whom he led up thither:

he charged them that they should tell no man what things they had
seen;
on the mount, as the transfiguration of himself, the persons of Moses and Elias, and the bright cloud from whence the voice came, which bore testimony of Christ's sonship: he ordered to keep the whole of this a secret from every man, even from their fellow disciples,

till the Son of man were risen from the dead;
(See Gill on Matthew 17:9).