Can the dawn (Subh) and sunset (Maghrib) prayers be shortened during travel?
Chapter on the Traveler's Prayer
Al-Mughni
Book of Prayer
Primary text
The consensus of scholars (Ijma' اهل العلم), as reported by Ibn al-Mundhir, is that shortening is not permitted for the Maghrib and Subh prayers. Shortening applies only to the four-unit prayers (Ruba'iyyat). The justification is that the Subh prayer is two units; shortening it would result in one unit, and the only prayer consisting of one unit is Witr (which is not applicable here), and Maghrib is the Witr of the day, so shortening it by one unit would nullify its nature as Witr or involve an unjust reduction by omitting most of it.
Supporting text
A narration attributed to Aisha indicates that the obligatory prayers were initially instituted as two units, except for Maghrib. After the Hijra, prayers were increased by two units, except for Subh (due to long recitation), Jumu'ah (due to the Khutbah), and Maghrib (as the Witr of the day). If one travels, one performs the prayer as originally instituted for them.