The World Federation One Stop Fiqh

Ruling 1350

If a traveller does not know some of the details of the prayer of a traveller – for example, he does not know that if he travels four farsakhs on his outward journey and four farsakhs on his return journey he must perform qaṣr prayers – then, in the event that he starts performing a prayer with the intention of performing a four rakʿah prayer and becomes aware of the ruling before going into rukūʿ of the third rakʿah, he must complete the prayer as a two rakʿah prayer. However, if he becomes aware of this in rukūʿ, his prayer is invalid based on obligatory precaution. And in case there is enough time for him to perform even one rakʿah, he must perform qaṣr prayers.