Textual: Even though Mormon is not specifically citing his source material, it is clear that this information is contained in the book of Nephi (4 Nephi to us). This is therefore the source for these events that Mormon referenced in 3 Nephi 28:19-22 where the events are listed in the same order, and with a similar vocabulary. Clearly the ultimate source underlying both instances is the same, and comes from the record in 4 Nephi. See the commentary on 3 Nephi 28:19-22 for more information.