[Minor Sins]

Answer – and Allah is the One who grants success and help: The scholars have differed regarding what minor sins are, after agreeing that there are minor and major sins.
The well-known view in the madhhab of our Imāms (Peace be upon them) is that every deliberate sin is a major sin. On this basis, minor sins are those sins that occur through mistake and forgetfulness. Among “mistake” is error in ijtihād and interpretation, as in the sin of Ādam and Yūnus (Peace be upon them).
Others have said: major sins are those for which Allah, exalted is He, has threatened (their doer) with the Fire, or has described them as “tremendous,” or those for which a prescribed punishment (ḥadd) has been legislated, and so on; and whatever is other than that is minor sins. And it has been said… and it has been said… etc.
I say: There is no doubt that a sin is described as “small” in relation to what is above it, and is described as “great” in relation to what is below it. So kissing a non-maḥram woman is greater than merely looking (at her), and smaller than fornication, and so it is with the rest of sins. However, one ought not to rely upon this method in defining minor sins, nor depend upon this scale, because of:
1. What came in the story of the Slander (al-ifk), when the Muslims passed along the story of the slander with their tongues and circulated it. Then Allah, exalted is He, rebuked them for that and condemned them, and said to them: “And you thought it insignificant, while it was, in the sight of Allah, tremendous.” [an-Nūr:15]
2. What came in the well-known ḥadīth, that the Prophet (May Allah bless him and his family and grant them peace) passed by two graves and said: “Indeed, they are being punished, and they are not being punished for something great. As for one of them, he used to walk about spreading talebearing, and as for the other, he did not protect himself from (the impurity of) urine.”
What appears correct to me is the statement of those who say that there are minor sins other than mistake and forgetfulness, but that Allah, exalted is He, did not define them. For if He, exalted is He, had defined them, that definition would amount to incitement from Him to commit them, and inciting toward what is evil is something from which Allah, exalted is He, must be declared far above.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3