Orson Pratt says that Joseph said polygamy was "a true principle" in 1831.
Orson Pratt, "Celestial Marriage," October 7, 1869, Journal of Discourses, 26 vols. (Liverpool: Horace S. Eldredge, 1871), 13:192-193
In the early rise of this Church, February, 1831, God gave a commandment to its members, recorded in the Book of Covenants, wherein He says, "Thou shalt love thy wife with all thy heart, and shalt cleave unto her and to none else;" and then He gives a strict law against adultery. This you have, no doubt, all read; but let me ask whether the Lord had the privilege and the right to vary from this law. It was given in 1831, when the one-wife system alone prevailed among this people. I will tell you what the Prophet Joseph said in relation to this matter in 1831, also in 1832, the year in which the law commanding the members of this Church to cleave to one wife only was given. Joseph was then living in Portage county, in the town of Hiram, at the house of Father John Johnson. Joseph was very intimate with that family, and they were good people at that time, and enjoyed much of the Spirit of the Lord. In the fore part of the year 1832, Joseph told individuals, then in the Church, that he had inquired of the Lord concerning the principle of plurality of wives, and he received for answer that the principle of taking more wives than one is a true principle, but the time had not yet come for it to be practised. That was before the Church was two years old. The Lord has His own time to do all things pertaining to His purposes in the last dispensation; His own time for restoring all things that have been predicted by the ancient prophets. If they have predicted that the day would come when seven women would take hold of one man, saying, "We will eat our own bread and wear our own apparel, only let us be called by thy name to take away our reproach;" and that, in that day the branch of the Lord should be beautiful and glorious and the fruits of the earth should be excellent and comely, the Lord has the right to say when that time shall be.