Luke 9:29
And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering.
And as he prayed, the fashion of his countenance was altered,.... It became exceeding bright and glorious, it shone like the sun,
Matthew 17:2 and hereby his prayer was answered; and thus, as Christ was heard and answered, whilst he was yet speaking, so are his people sometimes; and even their countenance is altered, when they are favoured with communion with God, and instead of a sad and dejected countenance, they have a cheerful one.
and his raiment was white and glistering; it was as white as the light, as snow, and whiter than any fuller on earth could whiten, as the other evangelists say, and so glistened exceedingly;
See Gill on "Matthew 17:2".
See Gill on "Mark 9:3".