What is the legally prescribed distance for shortening and combining prayers, and is it permissible to combine and shorten prayers when traveling a distance of 100 kilometers with a return on the same day, and what is the ruling on a traveler praying behind a resident imam?

Scholars differed on the distance for qasr (shortening prayers). Ibn Qudamah mentioned that the madhhab of Imam Ahmad is that qasr is not permissible for distances less than 16 farsakhs (approximately 84 km), and this is the view of Ibn Abbas, Ibn Umar, Malik, and Al-Shafi'i. It was also narrated from Ibn Umar that he performed qasr for a journey of 10 farsakhs (approximately 50 km), and from Anas that he performed qasr for 5 farsakhs (approximately 25 km).

Some scholars, such as Shaykh al-Islam Ibn Taymiyyah, preferred the view that qasr is permissible if the distance is considered a journey according to common custom and practice. Conditions and Rulings: Qasr is not permissible until the traveler has passed the houses of his village. It is permissible to combine Maghrib and Isha prayers, either by performing them earlier (jam' taqdim) or later (jam' ta'khir). If a traveler prays behind a resident imam, he must complete the prayer (i.e., not shorten it). Not performing qasr and jam' (combining prayers) does not invalidate the prayer. Shortening the four-rak'ah prayer is a sunnah (recommended practice) according to the majority of scholars, and combining prayers is permissible, though refraining from combining them is better.