Gospel Standards, pg. 22; “THE MISSION OF JESUS CHRIST. We are not dependent upon the testimony of men who lived many hundreds of years ago. We have the testimony that is given to us from the Prophet Joseph Smith, and from Sidney Rigdon that they saw the Son, that they heard the voice declare from the heavens that He was the Only Begotten of the Father, that by Him and of Him the worlds were created, and that He lives, for they saw Him.”
Romans 4:18, 23; “Who in hope believed against hope, to the end that he might become a father of many nations, according to that which had been spoken, So shall thy seed be. Now it was not written for his sake alone, that it was reckoned unto him.”
Also see Hebrews 11 that tells us we can use the saints of the OT as an example who serve as a “great cloud of witnesses” to what we are doing here.