According to John Tvedtnes, much of the book of Alma comes to us in the form of Mormon's abridgment, but Mormon occasionally quotes directly from Alma's record, usually marking it with a colophon, or prefacing remarks. Thus, for example, he introduces Alma's instruction's to his sons by saying, "We have an account of his commandments, which he gave unto them according to his own record" (Alma 35:16). The admonitions of each son are clearly the words of Alma and are marked as such by Mormon, who included a preface before each of the discourses (before Alma 36, 38, and 39). [John A. Tvedtnes, "The Voice of an Angel," in Book of Mormon Authorship Revisited, F.A.R.M.S., pp. 319-320]
Note* the phrases in these prefaces, "Comprising chapters 36 and 37" and "Comprising chapters 39-42 inclusive" are not the words of Mormon. They have been inserted by committees editing the Book of Mormon. They do not appear in the 1830 edition.