Prayer: A Traveller Mistaken About the Distance Performing Tamam Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
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 that after the time for prayers has expired, it is not necessary to make it up.