What should a man do who made a vow to Allah and was cured by Allah, but then forgot what this vow was?

If a person makes a vow and then doubts the type of vow, he should strive to identify its type and fulfill it. If he cannot determine a specific type with reasonable certainty, some scholars are of the opinion that he must perform all possible types, and this was favored by some Shafi'is. As for the majority of scholars, they believe that he must offer an expiation for a broken oath (kaffarat yamin); because doubting what was vowed is like not naming it, and it resembles one who vows and then becomes unable to fulfill it. They cite prophetic narrations, among them: "The expiation for a vow that was not specified is the expiation for an oath."