JOEL - In Hebrews we read:

4. And no man taketh this honour unto himself, but he that is called of God, as was Aaron.
5 So also Christ glorified not himself to be made an high priest; but he that said unto him, Thou art my Son, to day have I begotten thee.
6 As he saith also in another place, Thou art a priest for ever after the order of Melchisedec.
9. And being made perfect, he became the author of eternal salvation unto all them that obey him;
10 Called of God an high priest after the order of Melchisedec. (Heb 5:4-6, 9-10)

According to Joseph Smith, before the world was created, Jesus Christ, the great Jehovah and firstborn of God the Father in the spirit world, covenanted to use the power he had obtained from the Father to implement God's program for the eternal happiness of all God's children (cf. TPJS, p. 190)
So it appears from this and the Hebrews scripture, that Jesus received His priesthood authority from God the Father, which, according to Joseph Smith has "existed with God from eternity, and will to eternity, without beginning of days or end of years,…holding the keys of power and blessings." (TPJS, pp. 157, 322)
For us Christ Himself is therefore the source of all true priesthood authority and power on this earth, having received it from His father.