Praise be to Allah.
It was narrated from Ibn ‘Abbaas (may Allaah be pleased with them both) that the Prophet (peace and blessings of Allaah be upon him) said: “Vows are of two kinds: a vow that was for the sake of Allaah is expiated by being fulfilled, and that which was for the Shaytaan should not be fulfilled, and he has to offer expiation as in the case of kafaarat yameen (expiation for breaking a oath).”
Narrated by Ibn al-Jaarood in al-Muntaqaa, 935; al-Bayhaqi, 10/72.
This hadeeth was classed as saheeh by Shaykh al-Albaani in al-Silsilat al-Saheehah, 479.
Shaykh al-Albaani said:
[In this hadeeth] there is evidence of two things:
1 – That if a vow concerns an act of worship towards Allaah, then it must be fulfilled, and that is its expiation. It was narrated in a saheeh report that the Prophet (peace and blessings of Allaah be upon him) said:
“Whoever vows to do an act of obedience towards Allaah, then let him do it, and whoever vows to do an act of disobedience towards Allaah, then he should not do it.”
2 – Whoever swears a vow which involves disobeying the Most Merciful (Allaah) and obeying the Shaytaan is not permitted to fulfil it, and he has to offer expiation as in the case of kafaarat yameen [i.e., expiation for an unfulfilled oath]. If the vow concerns something makrooh (disliked), or he makes a vow to do something that is mubaah (permissible) [but he does not do it], then it is more apt that he should offer expiation, because of the general meaning of the words of the Prophet (peace and blessings of Allaah be upon him), “The expiation for a vow is kafaarat yameen.”
This was narrated by Muslim and others from the hadeeth of ‘Uqbah ibn ‘Aamir (may Allaah be pleased with him), and it is narrated in al-Irwaa’, 8/210.
What we have mentioned here, both the first and second points, is agreed upon by the scholars, except with regard to the obligation of offering kafaarah for vows concerning sin and so on. The view that it is obligatory is the view of Imaam Ahmad and Ishaaq, as al-Tirmidhi said (1/288). It is also the view of the Hanafis, and this is the correct view because of the hadeeth quoted above and what we have stated above.
Al-Silsilah al-Saheehah, hadeeth 479.