Helen Mar Kimball Whitney relates Joseph's account of an angel appearing to him and commanding him to practice plural marriage.
Helen Mar Kimball, Plural Marriage, As Taught by the Prophet Joseph. A Reply to Joseph Smith, Editor of the Lamoni (Iowa) "Herald" (Salt Lake City, UT: Juvenile Instructor, 1882), 13
This is but a small portion of the revelation [D&C 132], and Joseph's own testimony was, that an angel was sent to command him to teach and to enter into this order. This angel, he states, stood over him with a drawn sword prepared to inflict the penality of death if he should be disobedient.
I never doubted that young Joseph was exemplary, for he always had the reputation of being strictly moral, and I know that he was taught, as were many others, the first principles of the faith, which were all good and necessary for a foundation; but because we believe the precepts taught us in youth it does not excuse us for refusing to accept more revelation when given as the majority of the world have done—rejecting all revealed principles without investigation.