Discourses of Joseph Smith, pg. 270; “Do the Mormons believe in having more wives than one?” No, not at the same time. But they believe that if their companion dies, they have a right to marry again.”
Zechariah 13:2; “And it shall come to pass in that day, saith the LORD of hosts, that I will cut off the names of the idols out of the land, and they shall no more be remembered: and also I will cause the prophets and the unclean spirit to pass out of the land.”
At the time Smith made this announcement it was 1838. He was already practicing polygamy with three wives. Also see History of the Church 3:28-29.