Hebrews 5:6

And He says in another place [Psa. 110:4], "You are a priest forever according to the order of Melchizedek." [Note: This is the first mention of this mysterious Old Testament priest who, throughout this book, is represented as a type of Christ].

Hebrews 7:17

For it is declared [about Jesus in the Scriptures, Psa. 110:4], "You are a priest forever according to the order of Melchizedek."

Hebrews 7:21

(for [Levitical] priests were appointed without [God] taking [such] an oath, but Christ was appointed with an oath by God when He said to Him [Psa. 110:4], "The Lord made a vow and will not change His mind [when He said], 'You are a priest forever')."

Hebrews 5:10

[So], God declared Him to be a Head Priest according to the order of Melchizedek.

Hebrews 6:20

where Jesus, our forerunner, [already] entered on our behalf, having become a Head Priest forever according to the order of Melchizedek.

Hebrews 7:3

He was without a father, or mother or a list of ancestors [i.e., there was no record of his family tree] and no [record of his] birth or death. Like the Son of God, he [appeared to] remain a priest perpetually.

Hebrews 7:15

And this is even clearer if another priest like Melchizedek arises,

Treasury of Scripture Knowledge

Summary

As he saith also in another place, Thou art a priest for ever after the order of Melchisedec.

General references

Bible References

Thou

Hebrews 5:10
[So], God declared Him to be a Head Priest according to the order of Melchizedek.
Hebrews 6:20
where Jesus, our forerunner, [already] entered on our behalf, having become a Head Priest forever according to the order of Melchizedek.
Hebrews 7:3
He was without a father, or mother or a list of ancestors [i.e., there was no record of his family tree] and no [record of his] birth or death. Like the Son of God, he [appeared to] remain a priest perpetually.

General references

Hebrews 5:10
[So], God declared Him to be a Head Priest according to the order of Melchizedek.
Hebrews 7:17
For it is declared [about Jesus in the Scriptures, Psa. 110:4], "You are a priest forever according to the order of Melchizedek."