Fasting in the month of Ramaḍān and attending the Friday prayers continuously does not possess the strength to expiate all minor sins except if one also refrains from engaging in major transgressions. Thus, both actions together are able to expiate minor sin. How can the voluntary fasting of a single day expiate every single major sin perpetrated by a slave [of Allāh], and he continuously engages in this major sin, refusing to repent from it? This is impossible. This does not mean that fasting the day of ʿArafah or ʿĀshūraʾ does not expiate the sins of the entire year in general. Rather, these [aḥadīth] are from among the passages that pertain to a promise that is subject to prerequisites, prevented by certain clauses. [From among them] an individual who chooses to continuously engage in major sin prevents this expiation. If he then chooses to discontinue his major sin, his fasting along with this discontinuation are able to expiate [his minor sins] in general. Just as his [fasting] in Ramaḍān, and praying the five daily prayers, along with refraining from major sin, work hand-in-hand congruently to expiate his minor sin. Even though Allāh has said:
إِن تَجْتَنِبُوا كَبَائِرَ مَا تُنْهَوْنَ عَنْهُ نُكَفِّرْ عَنكُمْ سَيِّئَاتِكُمْ وَنُدْخِلْكُم مُّدْخَلًا كَرِيمًا ﴿٣١﴾
‘If you avoid the great sins which you are forbidden to do, We shall expiate from you your (small) sins, and admit you to a Noble Entrance (i.e. Paradise).’
It is known that just because a certain action has been specified as being a means of expiation, this does not prevent it from working in conjunction with another means to achieve this expiation. For this expiation would be stronger and more potent than if any one means is executed in isolation from the other. Hence, the means of expiation is strengthened [through engaging with a multitude of activities that promise expiation rather than a single one], Thus, the resultant expiation is stronger, more complete and comprehensive [of sin].
Source: Al-Dāʾ wa-al-Dawāʾ: 43-44
Translated by: Riyāḍ al-Kanadī