In June, 1829, the Lord called Oliver Cowdery, Martin Harris, and myself as the three witnesses, to behold the vision of the Angel, as recorded in the fore part of the Book of Mormon, and to bear testimony to the world that the Book of Mormon is true. I was not called to bear testimony to the mission of Brother Joseph Smith any farther than his work of translating the Book of Mormon, as you can see by reading the testimony of us three witnesses.

An Address to All Believers in Christ, page 32 (1887)

An Address to All Believers in Christ/Part Second/Chapter IV