«إن النذر نذران فما كان لله فكفارته الوفاء به وما كان للشيطان فلا وفاء له وعليه كفارة يمين»
The Prophet (peace and blessings be upon him) said:
"Indeed, vows are of two kinds: the vow made for Allah, its expiation is to fulfil it; and the vow made for Satan (i.e. for a sin) may not be fulfilled, and the expiation of an oath is binding upon him."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 1982