Moroni prophecies of the Three Witnesses in the Book of Ether.
Ether, Chapter 2 [Ether 5], Book of Mormon, Printer's Manuscript, ca. August 1829–January 1830, 435–436, The Joseph Smith Papers website, accessed February 13, 2023
And now I Moroni have written the words which was commanded me according to my memory & I have told you the things which I have sealed up therefore touch them not in order that ye may translate for that thing is forbidden you except by & by it shall be wisdom in God & behold ye may be privileged that ye may shew the plates unto those who shall assist to bring forth <forth this> work & unto three shall they be shewn by the power of God wherefore they shall know of a surety that these things are true & in the <mouth> of three witnesses shall these things be established & the testimony of three & this work inthewhich shall be shewn forth the power of God & also his word of which the Father & the Son & the Holy Ghost beareth record & all this shall stand as a testimony against the world at the last day & if it so be that they repent & come unto the Father in the name of Jesus they shall be received into the Kingdom of God & now if I have no authority for these things judge ye for ye shall know that I have authority when ye shall see me & we shall stand before God at the last day Amen.