According to the majority of jurists,

     he should do his best to remember what he has vowed by looking for any evidence or sign that might remind him of it, but if he can’t remember anything, he must make expiation for breaking an oath (kaffārat ul-yamīn).

     On the other hand, if he remembers that he has vowed to pray, fast or do some act of worship, but he cannot remember the number/amount of what he has vowed, he must perform the least of its type. For example, he can pray two Rak'ahs, fast one day and so on.