Brigham recorded his dream in which he, his legal wife, and the Prophet were traveling in a covered carriage: "Br Joseph Smith sat on the Back Seat with my wife [--] he whispered to hir [her--] Sead [said] it was wright [right] if she was a mind to [--] nothing more past [passed] betwen [sic] them." Then, in the dream, she disappears and is next seen as a corpse in a hearse.
During a meeting of Church President Wilford Woodruff with the apostles in the Salt Lake Temple on 25 June 1896, there was a discussion about "certain trials or tests to which Prests. B. Young and Jno. Taylor were put by Prest. Joseph the Prophet in Nauvoo, as the plurality & Eternity of the M. [Marriage] covenant was being revealed. Also what Emma was commanded to abstain from, and O. Hyde's trial also." Young's dream in December 1843 apparently referred to the Prophet's "test" of asking for Brigham's wife to become Joseph's.
[Brigham Young 1840-44 journal; Franklin D. Richards journal, 25 June 1896, LDS Church History Library; "Evidence For The Sexual Side of Joseph Smith's Polygamy," Comments by D. Michael Quinn on Session #2A "Reconsidering Joseph Smith's Marital Practices," Mormon History Association's Annual Conference, Calgary, Alberta, Canada, 29 June 2012 (unabbreviated version, revised during July)]