Is a traveler permitted to shorten the prayer if they commence travel after the prayer time has begun?
Chapter on the Traveler's Prayer
Al-Mughni
Book of Prayer
Primary text
The traveler is permitted to shorten the prayer (Qasr). This is the view held by Ibn al-Mundhir, who narrated consensus among scholars whose opinions are known. This position is adopted by Malik, al-Awza'i, al-Shafi'i, and the People of Opinion (Ahl al-Ra'y). The evidence provided is that the traveler departed before the time for the prayer elapsed, which is analogous to traveling before the prayer became obligatory.
Supporting text
An alternative view prohibits shortening the prayer because it became obligatory upon the person while they were a resident (muqim). Thus, they must complete it, similar to if they had traveled after the time elapsed or after commencing the prayer. This differs from traveling before the time began, as the obligation had not yet been established.