Brigham Young remembers being introduced to plural marriage in 1841 when he returned from his mission.
Brigham Young, "Secret of Happiness—Self Examination—Joseph Smith a Man of Obedience to God—Baptism for the Dead—Temporal and Spiritual One—a Dream—Order of Enoch, the Order of God—a Good Word for the Women," Journal of Discourses, 26 vols. (Liverpool: Joseph F. Smith, 1877), 18:241
We were driven from Missouri after Joseph went up there, and we came to Nauvoo, and the Twelve went to England. While we were in England, I think, the Lord manifested to me by visions and his Spirit, things that I did not then understand. I never opened my mouth to any persons concerning them, until I returned to Nauvoo. Joseph had never mentioned this, there had never been a thought of it in the Church that I knew anything about at that time. But I had this for myself, and I kept it to myself, and when I returned home and Joseph revealed these things to me, I then understood the reflections that were upon my mind while in England. But this was not until after I had told him what I understood. I saw that he was after something by his conversation, leading my mind along, and others, to see how we could bear this. This was in 1841; the revelation was given in 1843, but the doctrine was revealed before this; and when I told Joseph what I understood, which was right in front of my house in the street, as he was shaking hands and leaving me, he turned round and looked me in the eyes, and says he—"Brother Brigham, are you speaking what you understand,—are you in earnest?" Says I—"I speak just as the Spirit manifests to me." Says he—"God bless you, the Lord has opened your mind," and he turned and went off.