Revelation (west of Independence) for Oliver Cowdery, W. W. Phelps, Joseph Coe, and Ziba Peterson as they were about to commence a mission to Native Americans in Missouri, as recalled by W. W. Phelps in 1861: "For it is my will, that in time, ye should take unto you wives of the Lamanites and Nephites, that their posterity may become white, delightsome and Just, for even now their females are more virtuous than the gentiles." Since three of the four were already married, and the fourth--Ziba Peterson--would marry Rebecca Hooper within a month--this passage is sometimes used as an argument that plural marriage was envisioned as early as 1831.
Phelps recorded the revelation, apparently from memory,
Ezra Booth refers to this revelation in 1831. P Ezra Booth Letters (8-9)
[Kenney, Scott, Saints Without Halos, "Mormon History 1830-1844," http://web.archive.org/web/20120805163534/saintswithouthalos.com/dirs/d_c.phtml]