The World Federation One Stop Fiqh

Ruling 1278

If before travelling eight farsakhs one becomes unsure about continuing his journey, and while he is unsure he travels some distance and later decides to continue with his journey, then in the event that the total outward and return journey minus the distance he travelled while he was unsure is less than eight farsakhs, he must perform tamām prayers. If it is not less than eight farsakhs, his prayers must be in qaṣr form.