Prayer: Forgetting One is a Traveller and Performing Tamam Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1347
If a person forgets that he is a traveller and performs tamām prayers, in the event that he remembers this within the time for the prayer, he must perform the prayer in qaṣr form. If he remembers this after the time for the prayer, then making it up is not obligatory for him.