The World Federation One Stop Fiqh

Ruling 1270

With regard to someone like a prisoner who is travelling under the authority of someone else [i.e. a guard], in the event that he knows the journey will be eight farsakhs, he must perform qaṣr prayers. However, if he does not know, he must perform tamām prayers and it is not necessary for him to inquire [about the distance of the journey], although it is better that he does.