What is the ruling on someone who made a vow seventeen years ago to pray twenty rak'ahs, then prayed them fifteen years later, and then two years after that doubted whether he had performed them or not, with a strong presumption that he had performed them? Should he act upon this strong presumption or repeat the prayer?

Whoever vows obedience to Allah must fulfill their vow, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." If one is certain that they have fulfilled the vow, they should not pay attention to doubt, especially if they suffer from obsessive thoughts, to alleviate hardship and difficulty in the Sharia (definition). However, if they are doubtful about its performance, they must perform it.

A vow conditioned on a specific event does not become obligatory until that event occurs, and such a vow is disliked. As for an absolute vow not conditioned on anything, it is obligatory to fulfill it. If the vow was for a specific time and one delayed it without a legitimate excuse, they have sinned and must fulfill the vow along with an expiation for breaking an oath. But if the delay was due to an excuse, there is no sin and no expiation. If the vow is absolute and not time-bound, there is no harm in delaying it, but one should hasten to fulfill it.