Assalaamu alaykum. I have read many fatwas that state that delaying repentance is a sin necessitating repentance in and of itself. However, only statements of scholars were brought forth in this regard, without any proof from the Quran or Sunnah. Could you please bring the proof(s) which proves that delaying repentance is a sin? May Allah bless you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The proof that delaying repentance is a sin is that repentance becomes obligatory immediately upon committing a sin; Allah, the Exalted, says (what means): {O you who have believed, repent to Allah with sincere repentance.} [Quran 66:8]

There are many other texts that command Muslims to repent of their sins, and a command entails immediate compliance according to the scholars of Usool (Fundamentals of Fiqh). If a sinner failed to fulfill what he was commanded to do, which is immediate repentance, he bears a sin for it, and he is required to perform another repentance of the sin of delaying repentance. His only salvation of this is to perform a general repentance of all sins.

Ibn Al-Qayyim wrote:

"We shall shed light on some rulings related to repentance that the Muslim is in dire need of, and it is unbefitting of him to be ignorant of them. Immediate repentance of the sin is an immediate obligation, and it is not permissible to delay it. Whenever someone delays it, he has committed (another) sin by delaying it. If he makes repentance from his (original) sin, another repentance, which is repentance from delaying repentance, remains due on him. This is something that rarely comes to the mind of the repentant; rather, he thinks that if he has repented from the sin, there is nothing else due on him, while the duty to repent from delaying his repentance remains upon him. The only thing that can save him is to make a general repentance of what he knows of his sins and what he does not know. In fact, what one does not know of sins is more than what one knows. His ignorance of them does not exempt him from the obligation of repenting if he has religious knowledge, for in this case, he is sinful by neglecting the obligation of acting upon his knowledge and his sin is thus graver. The Prophet, sallallaahu ‘alayhi wa sallam, said, 'Indeed, Shirk (polytheism) is more subtle in this Ummah (community) than the creeping of an ant.' Abu Bakr asked, 'How can we be saved of it, O Messenger of Allah?' He, sallallaahu ‘alayhi wa sallam, said, 'That you say: Allahumma inni aʻoothu bika an ushrika bika wa ana aʻlam, wa astaghfiruka lima la aʻlam.' (O Allah, I seek refuge with You from knowingly associating anything with You, and I seek Your forgiveness for that which I do unknowingly.) [Ibn Hibbaan] This supplication contains seeking forgiveness for what Allah knows to be a sin while the person does not know it. Moreover, the Prophet, sallallahu ‘alayhi wa sallam, used to supplicate in his prayer, saying, 'Allahumma ighfir li khatee'ati wa jahli, wa israafi fi amri, wa ma Anta aʻlamu bihi minni. Allahumma ighfir li jiddi wa hazli, wa khata'i wa ʻamdi, wa kullu thaalika ʻindi. Allahumma ighfir li ma qaddamtu wa ma akh-khartu, wa ma asrartu wa ma aʻlantu, wa ma Anta aʻlamu bihi minni. Anta Ilaahi, la ilaha illa Anta.' (O Allah! Forgive my errors, ignorance, and immoderation in all my affairs, and what You are more aware of than me. O Allah! Forgive my faults which I committed in seriousness or in jest, deliberately or inadvertently, and all these (failings) are in me. O Allah! Forgive me for those sins which I committed in the past and which I may commit in the future, which I committed in privacy or in public and all those sins of which You are better aware than me. You are my Lord; none is worthy of worship but You.) He, sallallaahu ‘alayhi wa sallam, also used to say in his prostration, 'Allahumma ighfir li thanbi kullahu: diqqahu wa jillahu, khata'ahu wa ʻamduh, sirrahu wa ʻalaaniyatahu, 'awwalahu wa aakhirahu.' (O Allah! Forgive all my sins, the small and the great, those committed deliberately or inadvertently, those committed openly and those committed secretly, and the old and the new.) [Muslim] The general and comprehensive indication of these supplications make this a repentance of all the sins which the person knows and all those which he ignores."

Allah knows best.