Ruling when a traveler forgets a resident prayer and remembers it while traveling.

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If a traveler forgets a resident prayer (four rak'ahs) and remembers it while traveling, there is unanimous agreement that he must complete it as four rak'ahs. This is because the prayer was established upon him as four rak'ahs, and reducing this number is impermissible, just as if he had not traveled. He is merely making up what he missed, and what he missed was four rak'ahs.