1. The only first-person source comes from the epistle that Joseph Smith dictated on September 6, 1842, which was later canonized in the Doctrine and Covenants, Section 128.
The inference is that Joseph knew the name “Cumorah” before the book was revealed. That knowledge could only have come from Moroni. This is substantiated in the subsequent documents.
2. An early documentary source confirming the above, are the lines from a sacred hymn, written by W.W. Phelps. William Phelps lived with the Prophet in Kirtland and was in essence his executive secretary during the Nauvoo period.
And told that a record was hid in Cumorah,
Containing the fulness of Jesus’s gospel;
It was the angel who told Joseph that the record was hid in “Cumorah.” This hymn was selected by Emma Smith, wife of the Prophet, approved by the Prophet, and published in 1835 with a collection of hymns, under instructions and directions from the Lord. “And it shall be given thee, also, to make a selection of sacred hymns, as it shall be given thee, which is pleasing unto me, to be had in my church.” (D&C 25:1)
This hymn was also included in the 1841 edition as hymn #262.
3. Oliver Cowdery, Second Elder of the Church and Co-President with Joseph Smith, stated the following in 1831:
The Autobiography of Parley Parker Pratt was complied, edited and published in1881 by his son, from the documents and records left by his father after his death. From the length and detail of the address given by Oliver Cowdery in 1831, from which the above quote is taken, it had to have been recorded by Parley P. Pratt at the time it was spoken. “In writing his autobiography, Pratt relied heavily on his previous writings. After extensive analysis, Pratt family historian Steven Pratt concluded that almost ninety percent of the text is either based on or copied from earlier works”(Matt Grow, assistant professor of history at the University of Southern Indiana.)
4. The Prophet’s mother, Lucy Mack Smith, provides two separate items of evidence in the original manuscript of her memoirs. In the first item, Lucy is remembering what Joseph told her after Moroni first appeared to him. The quote begins with what Moroni had told Joseph:
Lucy dictated the above about 20 years after the fact, but it is consistent with other evidence. In the following, Lucy recalls directly what her son said in her presence. Following Joseph’s meeting with Moroni at Cumorah, one year before Joseph received the plates, Joseph told his parents that he had “taken the severest chastisement that I have ever had in my life.” Joseph said:
In both of these quotes from the Prophet’s mother, she demonstrates that in her mind it was Moroni, who told Joseph, prior to the translation of the plates, that the hill in Palmyra was named Cumorah.
5. David Whitmer confirmed this in an interview in his later years when he stated:
“An Angel from on high, The long, long silence broke – Descending from the sky, These gracious words he spoke: “Lo! in Cumorah’s lonely hill A sacred record lies concealed.””
How often have we sung this song without noticing that it was a quote from Moroni?
All of the documentary evidence is consistent that it was Moroni who told Joseph Smith, prior to the translation of the Gold Plates, that the ancient name of the hill in Palmyra was “Cumorah.” There is no evidence to the contrary.
h/t to Theodore Brandley
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Notice that proponents of the two-Cumorahs theory disbelieve all of this evidence, on top of Letter VII.
Source: Letter VII