Marriage to a mahram woman is a major sin but does not mean that one is an apostate
Question

                What is the ruling on a man who marries a woman who is not permissible for him, and he is aware of that, as some people deem him to be a disbeliever, quoting as evidence the hadith of al-Bara’ ibn ‘Aazib (may Allah be pleased with him) who said: My paternal uncle al-Haarith ibn ‘Amr passed by me carrying a banner that had been given to him by the Messenger of Allah (blessings and peace of Allah be upon him). I asked him and he said: The Messenger of Allah (blessings and peace of Allah be upon him) has sent me to strike the neck of a man who married his father’s wife.
Praise be to Allah.
	As far as we are aware, the scholars of the four madhhabs and 
	others are unanimously agreed that marriage to a mahram or to someone to 
	whom marriage is not permissible does not constitute apostasy in and of 
	itself, and it does not constitute disbelief that makes it permissible to 
	fight a person and seize his wealth. Rather it is one of the major sins and 
	shameful acts of disobedience to Allah. It does constitute disbelief and 
	apostasy if that person denies that it is prohibited and believes that this 
	marriage is permissible. 

	The scholars differed concerning the punishment of the one 
	who does that, but none of them suggested that he is to be executed for 
	apostasy. 

	According to the Hanafis, there is no hadd punishment for 
	that; rather he is to be subjected to a disciplinary punishment (ta‘zeer). 
	They interpreted the hadith mentioned by the questioner as referring to one 
	who regarded his action as permissible.

	Ibn al-Humaam al-Hanafi (may Allah have mercy on him) said:

	This indicates that he regarded that as permissible, and thus 
	became an apostate. End quote from Fath al-Qadeer (5/261)

	The majority of the Maalikis, Shaafa‘is and Hanbalis are of 
	the view that anyone who marries a woman who is his mahram should be 
	subjected to the hadd punishment for zina; if he was previously married then 
	he is to be stoned and if he was not previously married, then he is to be 
	given one hundred lashes. They also interpreted the hadith of al-Bara’ ibn 
	‘Aazib as referring to one who regarded this deed as permissible. 

	It says in at-Taaj wa’l-Ikleel, which is a Maaliki 
	book (8/370): 

	If he marries the wife of his father or the wife of his son, 
	he is to be subjected to the hadd punishment [for zina], if he was aware of 
	the prohibition on that. End quote.

	The Shaafa‘i scholar al-Maawardi (may Allah have mercy on 
	him) said:

	Because he regarded as permissible that which Allah, may He 
	be exalted, had definitively stated was prohibited, the Prophet (blessings 
	and peace of Allah be upon him) regarded him as an apostate, and by taking 
	the khums he classified his wealth as fay’ (a kind of booty).

	End quote from al-Haawi al-Kabeer (8/146). 

	The Hanbali scholar al-Bahooti (may Allah have mercy on him) 
	said:

	With regard to the report of al-Bara’ (about executing a man 
	and seizing his wealth), Abu Bakr said: According to Ahmad, it is to be 
	understood as referring to one who regarded that prohibited act as 
	permissible, and that the one who does not regard it as permissible is to be 
	treated as a zaani (fornicator or adulterer). 

	End quote from Kashshaaf al-Qinaa‘ (6/54-55). See 
	also: al-Furoo‘ by Ibn Muflih (10/56). 

	Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) 
	said:

	With regard to the hadith of Burdah ibn Nayyaar, according to 
	which the Prophet (blessings and peace of Allah be upon him) sent him to 
	someone who married his father’s wife, and he instructed him to strike his 
	neck and seize one fifth (khums) of his wealth, the fact that he seized one 
	fifth of his wealth indicated that he was a disbeliever, not just an 
	evildoer. He was deemed to be a disbeliever because he did not regard as 
	prohibited that which Allah and His Messenger had prohibited. 

	End quote from Majmoo‘ al-Fataawa (20/90) 

	What Shaykh al-Islam (may Allah have mercy on him) meant when 
	he said that “he did not regard as prohibited that which Allah and His 
	Messenger had prohibited” is that he regarded that act as permissible, as is 
	indicated by the words before and after that. 

	Shaykh al-Islam (may Allah have mercy on him) was asked about 
	a man who bought a slave woman and had intercourse with her, then he gave 
	her to his son. Is it permissible for his son to have intercourse with her? 

	He replied: 
It is not permissible for the son to have 
	intercourse with her after his father did so. This is according to the 
	consensus of the Muslims. Whoever thinks that that is permissible should be 
	asked to repent; if he repents, all well and good, otherwise he is to be 
	executed. In as-Sunan it is narrated that al-Bara’ ibn ‘Aazib said: I 
	saw my maternal uncle Abu Burdah carrying his banner and I said: Where are 
	you going? He said: The Messenger of Allah (blessings and peace of Allah be 
	upon him) has sent me to a man who married his father’s wife, and he has 
	instructed me to strike his neck and take one fifth of his wealth. There is 
	no disagreement among the leading scholars that it makes no difference 
	whether he had intercourse with her on the basis of marriage or on the basis 
	that she was his slave. 
	
	End quote from Majmoo‘ al-Fataawa (32/77) 

	Ash-Shawkaani (may Allah have mercy on him) said: 

	It is essential to interpret this hadith as meaning that this 
	man, who the Prophet (blessings and peace of Allah be upon him) issued 
	instructions was to be executed, was aware of the prohibition on that deed, 
	and he did it on the basis that he regarded it as permissible. This is 
	something that implies disbelief, and the apostate is to be executed. 

	End quote from Nayl al-Awtaar (7/137) 

	Ibn Hazm was of the view, which is also narrated from Ahmad, 
	that the punishment of one who has intercourse with a mahram should be more 
	severe than the punishment of one who commits zina. He must be executed in 
	any case, whether he was previously married or not. 

	However they said that he is to be executed as a hadd 
	punishment, not because of apostasy. 

	This view was favoured by al-Khattaabi and Shaykh al-Islam 
	Ibn Taymiyah. 

	Ibn Hazm (may Allah have mercy on him) said: 

	Whoever has intercourse with his father’s wife on the basis 
	of a contract that he calls marriage, or without such a contract, must 
	inevitably be executed, and seizing one fifth of his wealth is obligatory. 
	The rest of his wealth goes to his heirs – if he was not an apostate – or it 
	goes to the Muslims if he was an apostate. 

	End quote from al-Muhalla (12/204) 

	Ibn al-Qayyim (may Allah have mercy on him) said: 

	By Allah, (the Prophet (blessings and peace of Allah be upon 
	him)) did not merely rule that he was to be subjected to the hadd punishment 
	of zina; rather he ruled that his neck was to be struck and his wealth 
	seized. And this is the pure truth, for his crime was greater than that of 
	one who commits zina with his father’s wife without a marriage contract, for 
	such a one commits one sin, whereas the one who does the marriage contract 
	with her adds to the sin of intercourse the sin of a marriage contract that 
	Allah has forbidden. Thus he transgresses the sacred limits of divine law by 
	doing such a marriage contract and he transgresses the sanctity of his 
	mother by committing incest. 

	End quote from I‘laam al-Muwaqqi‘een (2/249). See 
	also: Zaad al-Ma‘aad (5/13-14); al-Mughni (9/56); Majmoo‘ 
	al-Fataawa by Shaykh al-Islam Ibn Taymiyah (34/177); Ma‘aalim as-Sunan 
	(3/329) 

	Conclusion: 

	Although this deed is emphatically prohibited and is a 
	serious crime, it does not constitute disbelief in and of itself, as is 
	indicated by the words of the scholars concerning the hadith quoted above, 
	unless the person who does it regards it as permissible, and does not submit 
	to the ruling of Allah and His Messenger concerning it. 

	And Allah knows best.