Reference: Fataawa Noor ‘alad-Darb – Question No.6112, Pages 530-531, Volume 11
Question: What is the ruling regarding undertaking a vow [or pledge] – is it haraam or halaal?
Response: Regarding undertaking a vow, [then] that which I am of the opinion is that it is haraam, and not permissible. This is because the Prophet (sal-Allaahu ‘alayhi wa sallam) prohibited that, and the base principle regarding a prohibition is impermissibility.
[In addition] it is also because it imposes upon the person something which Allaah has not imposed on him. [It is also] because he is vulnerable to not being able to carry out [this vow], particularly since some people undertake huge vows when their affair becomes extremely severe, and thereafter they find it difficult to fulfill the vow – and [no doubt] this is [a] very dangerous [affair].
Therefore, that which I lean towards and am of the opinion is that [the legal ruling regarding] undertaking a vow is haraam, and it is not something that is merely ‘disliked’. That is because the Prophet (sal-Allaahu ‘alayhi wa sallam) prohibited it, and [as I mentioned earlier] the base principle regarding a prohibition is impermissibility, not to mention the [unnecessary] hardship this person has imposed upon himself [by undertaking a vow].
 al-Bukhaaree – No.6234 and Muslim, No.1639