In Lucy Mack Smith, History, 1845, Lucy records what is the earliest prophecy of Joseph Smith (comments in square brackets added for clarification):
Joseph, during this [religious] excitement [in
1823], would say: that it would do us no injury to join them; for, if we did should, we would not continue with them long, for we were mistaken in them, and did not know the wickedness of their hearts. One day He said, he would give us an example; and, that we might set it down as a prophecy: thus, “you look at Deacon Jessup,’ said he, “and you hear him talk very piously; well, you think he is a very good man— Now, suppose that one of his poor neighbors should owe him the value of a cow; and, that this poor man had eight <little> children, and he should be taken sick and die,
leaving his wife with one cow but destitute of every other means of supporting them herself and family— Now I tell you, that Deacon Jesup, religious as he is, would not scruple to take the last cow from the poor widow, in order to secure the debt; notwithstanding he
himself has an abundance of everything.” This, at that time, seemed impossible; but one year had scarcely elapsed, when we saw the his prophecies prophecy litterally fulfilled.
Further Reading:
Resources on Joseph Smith's Prophecies