Mark 9:6
For he wist not what to say; for they were sore afraid.
Cross-reference
Mark 16:5-8 describes the women's terror and silence at the empty tomb—a similar fearful reaction to divine revelation.
Daniel 10:8 describes Daniel's strength failing in a vision — a similar terror at divine revelation, paralleling the disciples' fear here.
Matthew 17:4 records Peter's same offer to build tabernacles, showing his confusion — directly parallel to Mark 9:6.
Luke 9:33 says Peter spoke 'not knowing what he said' — the exact parallel to Mark 9:6's 'he did not know what to answer'.
Revelation 1:17 shows John falling as dead at the sight of the exalted Christ—a parallel to the disciples' terror at the Transfiguration.
Daniel 10:15-19 shows Daniel speechless from fear before an angel, paralleling Peter's inability to speak due to terror.