Is it permissible to shorten the prayer with a delayed intention after arriving at the destination, or is the intention required before commencing the journey?

For someone who embarks on a journey that permits the shortening of prayers (qasr), it is not a condition that they intend to shorten the prayer when they depart. Rather, according to some scholars, the intention to shorten the prayer is required when commencing the prayer. This is the view of the Shafi'is and the relied-upon opinion of the Hanbalis. Imam An-Nawawi stated in Al-Minhaj: "Intention (niyyah) for shortening [the prayer] at the time of ihram (commencement of prayer) is a condition."

However, some scholars hold that the intention to shorten the prayer is not a condition, and that a traveler may shorten their prayer even if they did not intend to do so. This is the view of the majority of scholars and was chosen by Sheikh al-Islam. All of Imam Ahmad's texts and responses regarding this matter are absolute, and it has not been reported from the Prophet (peace be upon him) nor from the Companions that they made the intention to shorten prayer a condition. If the prescribed prayer for the traveler is two rak'ahs, then performing them suffices, whether he intended to shorten it or not. This is the opinion of the majority, such as Malik, Abu Hanifa, and most of the early generations (Salaf). The texts are explicit that qasr is a fundamental principle that does not require an intention.

It must be noted that if a person's stay in a place is four days or more, they must pray the full prayer (itmam) from the moment they arrive and are not permitted to shorten it.