As Alma was journeying towards the city which was called Aaron, being weighed down with sorrow because of the wickedness of the people in the city of Ammonihah, an angel of the Lord appeared to him. He declared: "thou has great cause to rejoice; for thou hast been faithful in keeping the commandments of Gd from the time thou receivedst thy first message from him. Behold, I am he that delivered it unto you" (Alma 8:15; emphasis added).
Larry Dahl notes that apparently this was the same angel who appeared to Alma when Alma was converted. (See Mosiah 27) [Larry E. Dahl, "The Plan of Redemption," in Studies in Scripture: Book of Mormon, Part 1, p. 318]
“I Am He That Delivered It Unto You”
When Alma was weighed down and feeling like a failure in his mission to the people of the city of Ammonihah, an angel appeared to him and told him to rejoice. Interestingly, this angel told him that "thou hast great cause to rejoice; for thou hast been faithful in keeping the commandments of God from the time which thou receivedst thy first message from him. Behold, I am he that delivered it [the first message] unto you" (Alma 8:15). The account of that "first message" is found in Mosiah 27:11-16. According to Hugh Nibley, we used to teach much more about guardian angels in the Church. We used to teach much more of that doctrine. We don't do it anymore. I don't know why not, because it's a very real thing, the presence of another world. . . . Remember in Luke when the gospel was first being established, an angel went around and visited various people, namely Mary, the shepherds in the field, and Zacharias in the temple. It was the same angel; it was Gabriel. He said, I'm Gabriel. That was his particular mission, to introduce that dispensation of time. In the same manner, obviously, this angel was assigned to Alma. [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 2, p. 303]