Revelation, 23 July 1837 [D&C 112]

shall be damned. For unto you (the twelve) and those (the first  presidency) who are appointed with you to be your counsellors and  your leaders, is the power of this priesthood given for the last days  and for the last time, in the which is the dispensation of the fulness  of times: which power you hold in connection with all those who  have received a dispensation at any time from the beginning of the  creation, for verily I say unto you the keys of the dispensation which  ye have received have came down from the fathers; and last of all  being sent down from heaven unto you. Verily I say unto you, Behold  how great is your calling.
Cleanse your hearts and your  garments, lest the blood of this generation be required at your hands.  Be faithful untill I come for I come quickly and my reward is  with me to recompense every man according as his work shall be!  I am Alpha and Omega. Amen. [p. 74]
Revelation, Kirtland, OH, 23 July 1837; handwriting of James Mulholland; in JS, Journal, Mar.–Sept. 1838, pp. 72–74; CHL.