Doctrine and Covenants 58:43
43 By this ye may know if a man repenteth of his sins—behold, he will confess them and forsake them.
President Spencer W. Kimball wrote:
“Many offenders in their shame and pride have satisfied their consciences, temporarily at least, with a few silent prayers to the Lord and rationalized that this was sufficient confession of their sins. ‘But I have confessed my sin to my Heavenly Father,’ they will insist, ‘and that is all that is necessary.’ This is not true where a major sin is involved. Then two sets of forgiveness are required to bring peace to the transgressor—one from the proper authorities of the Lord’s Church, and one from the Lord Himself. [See Mosiah 26:29.]”
(Teachings of Presidents of the Church: Spencer W. Kimball , 41.)