The World Federation One Stop Fiqh

Ruling 1346

If a traveller who knows that he must perform qaṣr prayers performs tamām prayers supposing that his journey is less than eight farsakhs, then whenever he realises that his journey was eight farsakhs, he must perform the prayers he performed in tamām form again in qaṣr form. If he realises after the time for prayers has expired,  not necessary to make it up.