Repentance

The People of Knowledge have stated: It is necessary to repent from every sin. If the offense involves the Right of Allaah, not a human , then there are three conditions to be met in order that repentance be accepted by Allaah:

1- To desist from committing it.
2- To feel sorry for committing it
3- To decide not to recommit it

Any repentance failing to meet any of these three conditions, would not be sound.
But if the sin involves a human’s right, it requires a fourth condition, (i.e.,) to absolve oneself from such right. If it is a property, he should return it to its rightful owner. If it is slandering or backbiting, one should ask the pardon of the offended.
One should also repent from all sins. If he repents from some, his repentance would still be sound according to the people of sound knowledge . He should, however, repent from the rest. Scriptural proofs from the Book and the Sunnah and the consensus of the scholars support the incumbency of repentance.

Allaah, the exalted, says:

“And all of you beg Allaah to forgive you, O believers, that you may be successful. ” (24:31)

” Seek the forgiveness of your Rabb, and turn to him in repentance.” (11:3)

“O you who believe! Turn to Allaah with sincere repentance!”(66:8)
Al-Agharr bin Yasar Al-Muzani narrated that Allaah’s Messenger  Said, ” Turn you people in repentance to Allaah and beg pardon of him . I turn to him a hundred times a day.” (Muslim)

Ref:
Riyad-us-Saliheen pg20/chapter 2