Instruction on Priesthood, circa April 1835 [D&C 107]

1 There are, in the church, two priesthoods, namely: the  Melchizedek, and the Aaronic, including the Levitical priest hood. Why the first is called the Melchizedek priesthood, is  because Melchizedek was such a great high priest: before his  day it was called the holy priesthood, after the order of the Son  of God; but out of respect or reverence to the name of the Su preme Being, to avoid the too frequent repetition of his name,  they, the church, in ancient days, called that priesthood after  Melchizedek, or the Melchizedek priesthood.
2 All other authorities, or offices in the church are appenda ges to this priesthood; but there are two divisions, or grand  heads—one is the Melchizedek priesthood, and the other is the  Aaronic, or Levitical priesthood. [p. 82]
Instruction on Priesthood, Kirtland, OH, ca. Apr. 1835; in Doctrine and Covenants, 1835 ed., pp. 82–89.