SUMMARY:
Engaging in sexual intercourse with a menstruating woman is unanimously ḥarām in Islamic law.
This ruling is among those that are known by necessity in religion [maʿlūm fī al-dīn bi-al-ḍarūra], and thus, one who deems it lawful commits kufr.
The perpetrator must repent and seek forgiveness from Allah.
It is recommended by sunna that he give one gold dinar in charity (equivalent to approximately 4.25 grams of pure gold) to the poor and needy if the act occurred during the onset of menstruation, and half a dinar if it occurred during its cessation. This expiatory charity is incumbent upon the husband, not the wife.
Moreover, it is impermissible to derive sexual pleasure from the area between the navel and the knee during menstruation.
ANSWER:
In the name of Allah, the Most Gracious, the Most Merciful. All praise belongs to Allah, Lord of the Worlds. Prayers and salutations be upon the Master of the Messengers ﷺ, and upon his family and Companions. There is no power nor strength except through Allah, the Mighty, the Wise.
There is a consensus [ijmāʿ] among Muslims on the prohibition of intercourse with menstruating or postpartum women. One who deems this act permissible is out of the fold of Islam and commits kufr, based on the clear Qurʾānic āyāt and authentic Prophetic aḥādīth.
Allah, the Exalted, states:
“They will ask you about menstruation. Say, ‘It is an impurity, so keep apart from women during menstruation and do not approach them until they have purified themselves. But once they have purified themselves, then go to them in the way that Allah has enjoined on you.’ Allah loves those who turn back from wrongdoing and He loves those who purify themselves”
[Qurʾān 2:222].
And it is narrated on the authority of Anas (may Allah be pleased with him) that the Messenger of Allah ﷺ said:
“(During menstruation) do everything (with your wives) except intercourse.”
Narrated by Muslim §302
Additionally, it is narrated on the authority of Maymūna (may Allah be pleased with her) that she reported:
“Whenever the Messenger of Allah ﷺ intended to be physically affectionate with any of his wives during menstruation, he would instruct her to wear a waist-wrapper [izār] while she was menstruating.”
Narrated by al-Bukhārī §303; and Muslim §294.
Therefore, whoever engages in intercourse with a menstruating woman has committed a major sin. He must repent and seek forgiveness, and it is recommended that he give one dinar of pure gold (4.25g) in charity to the poor, and half a dinar if the act took place toward the end of the menstrual cycle. This is based on numerous reports, including those narrated by: Abū Dāwūd §264; al-Tirmidhī §136; al-Nasāʾī §289; Ibn Māja §650; al-Ḥākim, v. 1 p. 278; and others.
Among the Jews and Magians, menstruating women were entirely avoided, even refraining from sitting or eating with them. In contrast, Christians would not refrain from intercourse during menstruation. Islam, however, instituted a balanced path between these extremes.
It is not only permissible but recommended to live with, share meals with, and even lie beside one’s menstruating wife. Avoiding such intimacy, as the Jews did, is highly discouraged [makrūh]. However, one must strictly refrain from intercourse, which remains unlawful.
Enjoyment of the body between the navel and the knee during menstruation is prohibited, due to the preventive principle [sadd al-dharāʾiʿ]: granting unrestricted access to this area could lead to the prohibited act. As the legal maxim goes, “He who lingers near the forbidden is likely to fall into it.”
References:
See: Bidāya al-Mujtahid, v. 1 p. 65; al-Mughnī, v. 1 p. 243; al-Majmūʿ, v. 2 p. 359; Rawḍa al-Ṭālibīn, v. 1 p. 135; Talkhīṣ al-Ḥabīr, v. 1 p. 427; Fatḥ al-Qadīr, v. 1 p. 166; al-Zawājir, v. 1 p. 216; Mughnī al-Muḥtāj, v. 1 p. 280; Kashshāf al-Qināʿ, v. 1 p. 201; Ḥāshiya al-Dusūqī ʿalā al-Sharḥ al-Kabīr, v. 1 p. 173; Radd al-Muḥtār, v. 1 p. 297.