THE SECOND EPISTLE GENERAL OF PETER - Chapter 1 - Verse 18

Verse 18. And this voice which came from heaven we heard. To wit, Peter, and James, and John.

When we were with him in the holy mount. Called holy on account of the extraordinary manifestation of the Redeemer's glory there. It is not certainly known what mountain this was, but it has commonly been supposed to be Mount Tabor. See Barnes "Mt 17:1".