Albert Barnes Commentary 2 Peter 1:18

"and this voice we [ourselves] heard borne out of heaven, when we were with him in the holy mount." — 2 Peter 1:18 (ASV)

And this voice which came from heaven we heard. Namely, Peter, James, and John.

When we were with him in the holy mount. Called holy because 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 on Matthew 17:1.