I made a vow to Allah, the Exalted; I silently, without uttering the words, said, "I make a pledge to Allah to do such-and-such thing." Is that considered a valid vow?
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
When a person merely holds the intention to make a vow, thinks about it, plans to make it, or has inner thoughts about it without actually uttering the vow formula, then it is not incumbent on him to do anything in this regard. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Allaah Almighty has pardoned the members of my Ummah for the thoughts that occur to their minds, as long as such thoughts are not put into action or uttered." [Al-Bukhari and Muslim]
As for the case where the person says, "I make a pledge to Allaah to do such-and-such thing," some scholars held the view that this is considered an oath and that if he breaks it, he is obliged to offer expiation. This is the opinion of the Hanafi scholars and those who agreed with them on this issue.
The famous opinion of the Maaliki scholars in this regard is that it is not considered an oath.
Hence, if you had said by your tongue, "I make a pledge to Allaah to do such and such thing," specifying an act that is permissible, an act of obedience to Allaah, the Exalted, or an act that brings one closer to Him, but you failed to do it, then you do not have to offer an expiation, according to the Maaliki scholars. However, it is more prudent and cautious to offer the expiation if you broke your oath.
Allaah Knows best.
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