T. B. H. Stenhouse says he heard W. W. Phelps say that Joseph Smith first learned about polygamy translating the BOA papyri.
T. B. H. Stenhouse, The Rocky Mountain Saints (New York: D. Appleton Company, 1873), 182
Elder W. W. Phelps said in Salt Lake Tabernacle, in 1862, that while Joseph was translating the Book of Abraham, in Kirtland, Ohio, in 1835, from the papyrus found with the Egyptian mummies, the Prophet became impressed with the idea that polygamy would yet become an institution of the Mormon Church. Brigham Young was present, and was much annoyed at the statement made by Phelps, but it is highly probable that it was the real secret which the latter then divulged. The conscientious Mormon who calmly considers what is here written on the introduction of polygamy into the Mormon Church will readily see that its origin is probably much more correctly traceable to those Egyptian mummies, than to a revelation from heaven. The first paragraph of the Revelation has all the musty odour of the catacombs about it, and that Joseph went into polygamy at a venture there cannot be the slightest doubt.