Brigham says Joseph learned about plural marriage by a "manifestation" in 1830–1831; claims Oliver Cowdery at first accepted plural marriage.
Brigham Young, Speech, September 8, 1867, CR 100 912, CHL
...about this time little previous to this Joseph had a manifestation given to him that he didn't write out never was written until 1843 I think in the year 1830 or 1831 this manifestation was given to Joseph it was simply this the Lord said to Joseph the time has come that it is my will that my servants should take to themselves wives but the revelations [sic] was not given but a manifestation to Joseph that the elders of Israel would be required to take more than one wife Oliver Cowdery received this doctrine although Joseph told me says he Brother Brigham the Lord has revealed to me doctrine that I was to reveal to this church I hardly think a man tarry with me...