The World Federation One Stop Fiqh

Ruling 1342

With regard to someone who knows he is a traveller and must perform qaṣr prayers, if he intentionally performs tamām prayers in a place other than the four places mentioned above, his prayers are invalid. The same applies if he forgets that the prayers of a traveller are qaṣr and performs them in tamām form. However, if he remembers this after the time [for the prayer has expired], it is not necessary for him to make it up.