Ruling when a traveler forgets a traveling prayer (two rak'ahs) and remembers it while resident.

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If a traveler forgets a traveling prayer (Qasr) and remembers it while residing, Imam Ahmad, al-Awza'i, Dawud, and one opinion of al-Shafi'i hold that he must complete it as four rak'ahs as a precautionary measure. The justification is that shortening the prayer (Qasr) is a concession related to travel; thus, its validity ceases upon the removal of the cause, similar to wiping over socks for three days (which invalidates upon traveling). Furthermore, the prayer was obligated upon him as four rak'ahs while resident, evidenced by the statement, 'Let him pray it when he remembers it.' Finally, a ritual act that varies between residence and travel defaults to the ruling of residence when one of its timeframes is met.