Assalaamu alaykum. I made a vow to Allaah and broke this vow by accident as I forgot about it. As soon as I remembered, I stopped what I was doing that broke the vow and have not made the same mistake again. Do I have to pay an expiation as I broke the vow although I did so by mistake?

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, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Firstly, you did not mention what the vow that you made was about. If we assume that you had vowed not to do something and then did it unintentionally, then most of the scholars held that you do not have to pay kaffaarah (expiation). The Kuwaiti Encyclopedia of Fiqh reads, “The majority of Fiqh scholars held that if the person did what he had sworn not to do out of forgetfulness, this does not count as breaking the oath as long as it is not an oath of divorce or an oath to free a slave. The ignorant is held in the same standing of the forgetful in this regard; this is not considered breaking an oath...”

An-Nawawi wrote, “Scholars held two opinions pertaining to the case when one does or says what he swore not to do or say out of forgetfulness, ignorance, or under duress regardless of whether he had sworn by Allaah or had taken an oath of divorce. The most likely correct view of the two is that it does not count as breaking the oath.” [Rawdhat At-Taalibeen]

Shaykh Ibn ʻUthaymeen wrote:

“The correct view in this regard is that the same ruling applies to all these cases: an oath of divorce, an oath to free a slave, a vow, thihaar (saying to one's wife that she is like his mother's back i.e. unlawful to approach for enjoyment), or declaring a thing unlawful to oneself. All this is not considered breaking an oath unless the person has acted knowingly, attentive, and willful because breaking an oath denotes bearing a sin. So if you were ignorant or forgetful or were compelled, then you are not liable for expiation. This is the correct view, and it is the view adopted by Shaykhul-Islam Ibn Taymiyyah and the view supported by textual evidence with general indication.” [Ash-Sharh Al-Mumti'’]

If you repeated the action which you vowed not to do again afterwards while you were not forgetful (you did it intentionally) this time, then in such case, you are liable for expiation. The Prophet, sallallaahu ʻalayhi wa sallam, said, “The expiation of breaking a vow is the same as that of breaking an oath.” [Muslim and others]

The expiation of breaking an oath is to do one of the following three: feeding ten poor persons, or clothing them, or freeing a slave. Whoever is unable to do any of the three, then he should fast three days.

It should be noted that it is disliked to make vows. Ibn Qudaamah wrote:

“It is not recommended to make vows; it has been narrated on the authority of Ibn ʻUmar that the Prophet, sallallaahu ʻalayhi wa sallam, said, 'The vow neither hastens anything nor defers anything, but is the means by which something is extracted from the miser.' [Al-Bukhari and Muslim] The prohibition in this regard denotes dislike rather than forbiddance, because if it were forbidden, those who fulfill their vows wouldn't have been praised (as in verse 76:7) because their sin in making the vow would have been considered graver than their obedience in fulfilling it. Moreover, if making vows were recommended, the Prophet, sallallaahu ʻalayhi wa sallam, and his noble Companions would have made vows.” [Al-Mughni]

Allaah knows best.