Jacob says God commanded Lehi to practice monogamy.
Jacob 3:5–6, The Book of Mormon, The Church of Jesus Christ of Latter-day Saints, accessed May 30, 2023
5 Behold, the Lamanites your brethren, whom ye hate because of their filthiness and the cursing which hath come upon their skins, are more righteous than you; for they have not forgotten the commandment of the Lord, which was given unto our father—that they should have save it were one wife, and concubines they should have none, and there should not be whoredoms committed among them.
6 And now, this commandment they observe to keep; wherefore, because of this observance, in keeping this commandment, the Lord God will not destroy them, but will be merciful unto them; and one day they shall become a blessed people.