The great scholar of Islam, Ibn Qayyim al-Jowziyyah [d.752] (may Allaah have mercy on him) said:

“…And we shall mention some things related to repentance (towbah) that are very necessary (to grasp), and it is not befitting that someone be ignorant of them.

From them: Immediate repentance is obligatory after committing a sin. It is not permissible to delay it. Whenever someone delays it, he has committed (another) sin by delaying it. In this case, when he repents from the sin (itself), another act of repentance remains a duty upon him – that he must repent for delaying his repentance! This is something that rarely comes to mind when someone repents. Instead, he may assume that since he has repented from the sin, that there is nothing else left to do, while there remains upon him the duty to repent from delaying his repentance…”