His statement, “and the Day of Sacrifice”: that is, if someone says, “I owe Allah a vow that I will fast on the Day of Sacrifice,” we say: This is prohibited and an act of disobedience. Is there a more inclusive expression than the author’s phrase, “the Day of Sacrifice”?

The answer is: Yes—the two days of Eid, for the day of Eid al-Fitr is like the Day of Sacrifice. There are also other days on which fasting is prohibited: the Days of Tashriq. In short, if a person vows to fast on a day when fasting is prohibited, then this is a vow of disobedience that must not be fulfilled.

Likewise, if he vows not to speak to his paternal uncle, this is a vow of disobedience whose fulfillment is prohibited.

His statement, “he must not fulfill it,” is based on the Prophet’s ﷺ saying: “Whoever vows to disobey Allah, let him not disobey Him.” (1) Moreover, if it were permissible to fulfill such a vow, then everyone who wished to commit a sin could simply make a vow to do it. This would lead to violating the sacred limits set by Allah.

His statement, “and he must offer expiation,” means that he must offer the expiation for an oath, based on the Prophet’s ﷺ saying: “There is no vow involving disobedience to Allah, and its expiation is the expiation for an oath.” (2) Imam Ahmad, Isaac ibn Rahwayh, and al-Tahawi—who judged it authentic—cited this hadith as evidence, contrary to al-Nawawi’s statement that it is weak by the consensus of the hadith scholars. Yet Imam Ahmad cited it as evidence, and his citing it indicates that he regarded it as authentic. Al-Tahawi likewise authenticated it, and he is among the imams whose authentication carries weight. Accordingly, the hadith is authentic and may be cited as proof.