Assalaamu alaykum. What is hurmat musaharah (nullifiction of marriage due to, for example, a mother touching her son with lust)? This is said to nullify the marriage of his parents. What proof is there for this?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Stating that touching with lust entails hurmat musaaharah (being premanently forbidden in marriage) was the view of the Hanafi scholars, contrary to the majority of the scholars, who held that touching with lust does not entail hurmat musaaharah.

The Hanafi jurists relied on some texts to support their view, some of which were cited by the Hanafi scholar Al-Kaasaani who wrote:

"It has been narrated that the Prophet, sallallaahu ʻalayhi wa sallam, said, ‘Whoever looks at the private parts of a woman, her mother and daughter are no longer lawful to him (in marriage).’ Another version of the report reads, ‘Whoever looks at the private parts of a woman, his marriage to her mother and daughter becomes prohibited.’ This hadeeth is primary evidence in this regard because the Prophet, sallallaahu ʻalayhi wa sallam, did not mention intercourse (only looking).

It was also reported that the Prophet, sallallaahu ʻalayhi wa sallam, said, ‘Cursed is the one who looks at a woman's and her daughter's private parts.’ If the first look does not entail deeming the second prohibited (i.e. looking at the private parts of the daughter), then the doer would not be subject to the curse because it is lawful for the man to look at the private parts of his wife to whom his marriage is valid as per the sharee'ah; then how come the Prophet, sallallaahu ʻalayhi wa sallam, would state that the man in reference would be cursed for it? If the prohibition of marriage to the woman's daughter and mother is established by looking at her private parts (wife), then it is established by the consummation of marriage with greater reason because looking is subordinate to touching as far as the religious rulings are concerned...” [Badaa‘i’ As-Sanaa‘i’]

It should be noted though that the ahaadeeth about declaring the woman unlawful to the man in case of lustful touching or looking are inauthentic according to the scholars of hadeeth. Ibn Hajar wrote, “There is a weak report in this regard cited by Ibn Abi Shaybah on the authority of Umm Haani' that says that the Prophet, sallallaahu ʻalayhi wa sallam, said, ‘Whoever looks at the private parts of a woman, her mother and daughter become unlawful to him.’ [Marfoo’, i.e. attributed to the Prophet, sallallaahu 'alayhi wa sallam ] The chain of narration of this report is unknown according to Al-Bayhaqi.”

The Hanafis summed up the establishment of the prohibition of marriage in this context in Al-Ikhtiyaar li-Taʻleel Al-Mukhtaar as follows:

“Adultery entails the establishment of the prohibition of marriage; whoever commits adultery with a woman or has intercourse with her mistakenly believing it is lawful, her female descendants and ascendants become unlawful to him. Moreover, this woman with whom he commits adultery, becomes unlawful to his male descendants and ascendants as well. The same applies to a man who touches a woman or even looks at her with lust; marriage to any of her female descendants and ascendants becomes prohibited for him, and marriage to the male descendants and ascendants of the man in reference becomes prohibited for her.”

Hence, when a woman touches a man with lust, the man's father becomes unlawful to her. Therefore, if a woman touches her son with lust, his father (i.e. her husband) is declared unlawful to her (according to the Hanafis). However, the majority of scholars held a contrary opinion, as has been mentioned above.

Allaah knows best.