Al-Nawawi writes:

The scholars said it is an obligation to repent from every sin. If it is sinful disobedience between a servant and his Lord – unrelated to the right of a human being – then it has three conditions. First, the disobedience is abandoned. Second, one regrets what they have done. Third, one is determined not to repeat it ever again. If one of these three is missing, his repentance is not sound.

Al-Nawawi continues:

If it is sinful disobedience related to the right of a human being, there is a fourth condition alongside these three. It is that he acquits himself of the right by restoring it to its owner, if it is related to wealth or otherwise.

Source: Riyāḍ al-Ṣāliḥīn 1/14