[Praying over a Deceased Person Who Was Lax about Some Sins]

Answer—and Allah grants success: The school’s position, as in the marginalia upon al-Azhār, is that the funeral prayer and washing are obligatory for one who is in the ruling of a sinner (fāsiq)—one whose integrity (ʿadālah) is compromised. Ended with its meaning.
I say: What appears to me is that “one in the ruling of a sinner” means someone who is lax regarding sins for which there is no decisive proof that they are major sins. The proof is their established principle: “It is impermissible to declare someone an unbeliever or a sinner except with decisive proof.”
Accordingly, the funeral prayer and washing are obligatory for any Muslim who dies without it being known that he died persisting upon a major sin.
This may also be inferred from Allah’s saying to His Prophet (May Allah bless him and his family and grant them peace): “So it was by mercy from Allah that you were gentle with them. Had you been harsh and hard-hearted, they would have dispersed from around you. So pardon them, ask forgiveness for them, and consult them about the matter.” [Āl ʿImrān:159] Allah, Exalted is He, commanded His Prophet (May Allah bless him and his family and grant them peace) to pardon and seek forgiveness for Muslims from whom some acts of disobedience and contraventions issued.
Muslims were not all at one level of knowledge of Allah, His rulings, and His etiquettes. Among them were a few whose knowledge was firm; others below that; coarse desert dwellers; the ignorant; and those of weak faith. Their degrees of piety differed; therefore, contraventions and sins were bound to occur among them—an inevitable result in any society marked by such differing traits, whether in the time of the Prophet (May Allah bless him and his family and grant them peace) or otherwise.
Thus Allah commanded His Prophet to meet those outcomes with pardoning and seeking forgiveness—and then with consultation, which signals approval, desire, love, and closeness.
However, we say that such noble conduct is conditioned and limited by His saying about the hypocrites in Sūrat al-Tawbah: “And never pray [the funeral prayer] over any of them who dies, and do not stand at his grave.
Surely they disbelieved …” [al-Tawbah:84], and by His saying in the same sūrah: “It is not for the Prophet and those who have believed to ask forgiveness for the polytheists—even if they were relatives—after it has become clear to them that they are companions of the Blaze. And the request of Abraham for forgiveness for his father was only because of a promise he had made to him; but when it became clear to him that he was an enemy to Allah, he disassociated himself from him …” [al-Tawbah:113].
Thus, it is impermissible to seek forgiveness or to pray over one who has become clear to be among the companions of the Blaze, or who has become clear to be an enemy of Allah, Exalted. Whoever is established with certainty to be among the companions of the Blaze, it is impermissible to pray over him; and the people of major sins are among the companions of the Blaze with certainty.
By “major sins” here we mean: those for which there is certain threat of the Fire—such as associating partners with Allah, killing a soul forbidden [to be killed], fleeing from battle, falsely accusing a chaste woman, fornication, consuming usury, and the like—matters about whose gravity there is unanimous agreement.
As for matters about whose being a major sin there is disagreement among the scholars of Islam, such disagreement does not prevent praying over the deceased, even if in our view it is a major sin. That is due to what has preceded of His saying, Exalted is He: “after it has become clear to them that they are companions of the Blaze,” [al-Tawbah :113] and His saying: “When it became clear to him that he was an enemy to Allah, he disassociated himself from him.” [al-Tawbah:114] “Becoming clear” (tabayyun) only applies where knowledge has become firmly established and clarified to the utmost. From this is derived the proof for their statement: that it is impermissible to declare someone an unbeliever or a sinner except with decisive evidence.
If it is said: There is a ḥadīth reported from the Prophet (May Allah bless him and his family and grant them peace) commanding prayer over anyone who says “Lā ilāha illā Allāh,” and to pray behind anyone who says “Lā ilāha illā Allāh.”
We say: The ḥadīth is not authentic according to the ḥadīth scholars, as al-Jalāl indicated in Ḍawʾ al-Nahār. Then we further say: even if we assumed it authentic, it is a general text specified by what has preceded. Its sense would then be: “Pray behind and over whoever says ‘Lā ilāha illā Allāh’—except one who has become clear to be among the companions of the Blaze, or one who has become clear to be an enemy of Allah; do not pray over him nor behind him.”
If it is said: What you have cited of verses concerns polytheists; that is not our discussion. Our discussion is about sinful monotheists, who are not mentioned in the verses.
We say: The reasoning (taʿlīl) in the two verses—namely, “after it has become clear to them that they are companions of the Blaze” [al-Tawbah :113] and “When it became clear to him that he was an enemy to Allah, he disassociated himself from him” [al-Tawbah:114]—indicates the generality of the ruling for anyone in whom this effective cause is realized, even if the initial ruling is about a specific group. This is like a physician saying to a patient: “Do not drink honey because of its sweetness.” The patient—and any rational person—understands from such wording that the patient ought not to eat sugar or drink sweet juices. Usul scholars differ here on two statements:
 One group says: the prohibition of sugar, sweet juice, and any other sweet thing is encompassed by the generality of the ʿillah (the effective cause); thus every sweet thing is prohibited by the text, not by analogy.
 The second group says: the prohibition in such a case is specific and does not encompass every sweet thing; it applies only to honey. When we judge sugar, sweet juices, and the like to be prohibited, that is by analogy—we have analogized every sweet thing to honey, the unifying cause being sweetness.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1