John Gill Commentary Hebrews 7:17

"for it is witnessed [of him,] Thou art a priest for ever After the order of Melchizedek." — Hebrews 7:17 (ASV)

For he testifies
That is, either David, the penman of the psalm, or rather the Holy Ghost, the enditer of it, or God in the Scripture, in (Psalms 110:4) of this form of citing Scripture, (See Gill on Hebrews 2:6).

you are a priest for ever after the order of Melchisedec ;
see (Hebrews 5:6Hebrews 5:10) (6:20) .