8 And now, behold, I say unto you that you shall go unto the Lamanites and preach my gospel unto them; and inasmuch as they receive thy teachings thou shalt cause my church to be established among them; and thou shalt have revelations, but write them not by way of commandment.
9 And now, behold, I say unto you that it is not revealed, and no man knoweth where the city Zion shall be built, but it shall be given hereafter. Behold, I say unto you that it shall be on the borders by the Lamanites.
He [the angel] said there was a book deposited, written upon gold plates, giving an account of the former inhabitants of this continent, and the source from whence they sprang.
The Wentworth Letter, by Joseph Smith Jr.
… I was also informed concerning the aboriginal inhabitants of this country [America] and shown who they were, and from whence they came; a brief sketch of their origin, progress, civilization, laws, governments, of their righteousness and iniquity, and the blessings of God being finally withdrawn from them as a people, was [also] made known unto me; I was also told where were deposited some plates on which were engraven an abridgment of the records of the ancient prophets that had existed on this continent. …
… In this important and interesting book the history of ancient America is unfolded, from its first settlement by a colony that came from the Tower of Babel at the confusion of languages to the beginning of the fifth century of the Christian era. We are informed by these records that America in ancient times has been inhabited by two distinct races of people. The first were called Jaredites and came directly from the Tower of Babel. The second race came directly from the city of Jerusalem about six hundred years before Christ. They were principally Israelites of the descendants of Joseph. The Jaredites were destroyed about the time that the Israelites came from Jerusalem, who succeeded them in the inheritance of the country. The principal nation of the second race fell in battle towards the close of the fourth century. The remnant are the Indians that now inhabit this country. …
Also see Fuzzy_Thoughts’s discussion on the use of this statement in lesson material and how the Wentworth letter has traditionally been viewed in correlated material.
Acknowledgements: reddit users
Obadiah_Dogberry pointed out some of these sources in this post.