Ruling on making a vow to do something and then forgetting it
9-8-2015 | IslamWeb
Question:
A man made a vow to Allah, the Exalted, that if his father recovered from his illness, he would do such-and-such a thing, but he later forgot what he had vowed to do. What should he do?
Answer:
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.
If someone made a vow and then became doubtful or forgot about it—whether it was a vow to pray, fast, offer charity, or the like—he should strive to remember it until he becomes almost certain about what he had vowed to do. If he strove to remember what he had vowed to do but failed to identify it, some scholars believed that he should do all that he suspects to have vowed to do (prayer, fasting, offering charity, or the like). This opinion is favored by a group of Shaafi‘i scholars such as Imaams Ar-Ramli and Al-‘Abbadi 
. They said, "He can only clear his liability from this commitment with certainty if he does all that he suspects to have vowed to do. This is because anything without which an obligation cannot be fulfilled is in itself obligatory."
The majority of the scholars believed that the person, in this case, is required to offer expiation like that for breaking an oath because doubting what he had vowed to do is the same as making a non-specific vow. The person who fails to specify what he had vowed to do is precisely in the same situation as the one who makes a vow and then failed to deliver on it. The Prophet, sallallaahu ‘alayhi wa sallam, said: "The expiation for a vow when it is not specified is the expiation for (breaking) an oath." [At-Tirmithi]
Moreover, Ibn ‘Abbaas 
reported that the Prophet, sallallaahu ‘alayhi wa sallam, said:
"If anyone makes a vow but does not name it, its expiation is the same as that for (breaking) an oath. If anyone makes a vow to do an act of disobedience, its expiation is the same as that for (breaking) an oath, if anyone makes a vow that he is unable to fulfill, its expiation is the same as that for (breaking) an oath, but if anyone makes a vow that he is able to fulfill, he must do so." [Abu Daawood]
Allaah Knows best.