then let him not disobey Him.” (1) Therefore, it is not permissible to fulfill such a vow.
If he says, “I owe Allah a vow that I will strike so-and-so,” without any cause, we say: Offer expiation and do not do it. If the person says to him, “Strike me,” then it becomes permissible, since he has waived his right; but in principle, it is a vow to commit a sin.
If a man vows to engage in usury, we say: It is forbidden for him, and he must offer the expiation for an oath. It is forbidden for him because of the Prophet’s ﷺ statement: “Whoever vows to disobey Allah, then let him not disobey Him.” And he must offer the expiation for an oath because of his ﷺ statement: “There is no vow involving disobedience to Allah, and its expiation is the expiation for an oath.” (2)
His statement, “fasting on a day of menstruation,” refers, for example, to a woman who says, “I owe Allah a vow that I will fast on the first day of my menstruation.” We say: This is forbidden, because it is forbidden for a menstruating woman to fast, by the consensus of the Muslims, based on the Messenger’s ﷺ statement: “Is it not the case that when she menstruates, she neither prays nor fasts?” (3) Thus, it is not possible for this woman to fast on a day of menstruation.
(1) This is the wording of a hadith recorded by al-Bukhari in al-Ashribah (Drinks), under the chapter “Allah’s saying: ‘Intoxicants, gambling, and sacrificial altars …’” (no. 5575), and by Muslim in al-Ashribah, under the chapter “Clarification That Every Intoxicant Is Khamr” (no. 2003), with the additional wording, “then he does not repent from it,” in al-Bukhari’s version, and, “unless he repents,” in Muslim’s version. The hadith is narrated from Ibn Umar.
(2) Recorded by Ahmad (6:247); al-Nasai, in al-Ayman wa-al-Nudhur (Oaths and Vows), under the chapter “Expiation for a Vow” (7:26); Abu Dawud, in al-Ayman wa-al-Nudhur, under the chapter “Who Holds That Expiation Is Required” (no. 3290); al-Tirmidhi, in al-Nudhur wa-al-Ayman (Vows and Oaths), under the chapter “What Has Been Reported from the Messenger of Allah, peace and blessings be upon him, That There Is No Vow …” (no. 1525); Ibn Majah, in al-Kaffarat (Expiations), under the chapter “A Vow of Disobedience” (no. 2125); al-Hakim (4:339); and al-Tahawi (3:42). See also al-Talkhis al-Habir (4:175–76). It was graded sahih by al-Albani in Irwa al-Ghalil (no. 2590).
(3) Recorded by al-Bukhari in al-Hayd (Menstruation), under the chapter “A Menstruating Woman’s Abstention from Fasting” (no. 304), and by Muslim in al-Iman (Faith), under the chapter “Clarification That Faith Decreases through Deficiencies in Acts of Obedience” (no. 80), on the authority of Abu Said al-Khudri. This is al-Bukhari’s wording.