History of the Church, Joseph Smith 5:402

The Lord hath set the bow in the cloud for a sign that while it shall be seen, seed time and harvest, summer and winter shall not fail; but when it shall disappear, woe to that generation, for behold the end cometh quickly.

History of the Church, Joseph Smith 6:254

I have asked of the Lord concerning His coming; and while asking the Lord, He gave a sign and said, “In the days of Noah I set a bow in the heavens as a sign and token that in any year that the bow should be seen the Lord would not come; but there should be seed time and harvest during that year: but whenever you see the bow withdrawn, it shall be a token that there shall be famine, pestilence, and great distress among the nations, and that the coming of the Messiah is not far distant.