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Source:Doctrine and Covenants:132:51-52:all those that have been given unto my servant Joseph
Doctrine and Covenants 132:51-52: "all those that have been given unto my servant Joseph"
D&C 132 (Latter-day Saint scripture):
51 Verily, I say unto you: A commandment I give unto mine handmaid, Emma Smith, your wife, whom I have given unto you, that she stay herself and partake not of that which I commanded you to offer unto her; for I did it, saith the Lord, to prove you all, as I did Abraham, and that I might require an offering at your hand, by covenant and sacrifice.
52 And let mine handmaid, Emma Smith, receive all those that have been given unto my servant Joseph, and who are virtuous and pure before me; and those who are not pure, and have said they were pure, shall be destroyed, saith the Lord God. (DC 132:51-52)
Since it speaks of those "that have been given unto my servant Joseph," this clearly indicates that Joseph was practicing plural marriage.