The Messenger of Allah ﷺ said, "The expiation for an unfulfilled vow is the same as the expiation for a broken oath."
عن رسول الله ﷺ قال: كفارة النذر كفارة اليمين.
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Muslim narrated it in the Book of Vows (1645) through the route of Ibn Wahb, who said, Amr bin Harith told me, from Kaab bin Alqama, from Abdul-Rahman bin Sahammasa, from Abu Khayr, from Uqba bin Amir, and he related it.Those who held that there is an expiation for sin in general relied on this unqualified hadith.Tirmidhi narrated it (1528) through another route, from Abu Bakr bin Ayyash, who said, Muhammad, the freedman of Mughira bin Shuba, told me, who said, Kaab bin Alqama told me, from Abu Khayr, from Uqba bin Amir, who said, the Messenger of Allah ﷺ said, "The expiation for a vow, when it is not specified, is the expiation for an oath."In this chain is Muhammad, the freedman of Mughira, who is Muhammad bin Yazid bin Abu Ziyad Thaqafi, "unknown."Abu Dawud also narrated it (3323) through his route, but without the words "when it is not specified."Ibn Majah narrated it (2127) through another route, from Ismail bin Rafi, from Khalid bin Yazid, from Uqba bin Amir, and he mentioned in it, "when he did not specify it."Ismail bin Rafi Ansari Madani is daif in memory, as stated in Taqrib.The meaning of his statement, "when it is not specified," is that the expiation of an oath is only required for vows that were left unspecified. Scholars have applied this qualified version to the unqualified version found in the hadith of Uqba bin Amir as narrated by Muslim.