The Doctrine and Covenants, Section 88:105

105 And again, another angel shall sound his trump, which is the sixth angel, saying: She is fallen who made all nations drink of the wine of the wrath of her fornication; she is fallen, is fallen!

Journal of Discourses, Wilson Woodruff 21:22

Joseph Smith, I expect, will sound the sixth trumpet. He will be at the head of this dispensation; or, if he does not blow the trumpet of this dispensation, I do not know who will. Somebody has got to do it, and it must be somebody holding the keys of the various dispensations of the world. No other angels are coming from any other world to administer in this dispensation; those men will minister who dwelt here in the flesh.