Ruling on Shortening Four-Unit Prayers for the Traveler
Is shortening the prayer for a traveler mandatory or optional?
Hanafi
Shortening four-unit prayers is obligatory for the traveler; completing them is prohibitively disliked provided he sat for the first sitting (otherwise his obligatory prayer is not valid).
Reference: Radd al-Muḥtār — Al-ʿInāyah — Marāqī al-Falāḥ
Maliki
Shortening four-unit prayers during travel is an emphasised sunnah, superior to completing them.
Reference: Al-Sharḥ al-Kabīr — Al-Tāj wa-al-Iklīl — Minaḥ al-Jalīl
Shāfiʿī
Shortening is a permitted concession for the traveler once his journey reaches two stages; completing the prayer is better, unless it reaches three stages or more, in which case shortening is better.
Reference: Minhāj al-Ṭālibīn — Al-Majmūʿ — Mughnī al-Muḥtāj
Hanbali
Shortening is a concession and is superior to completing; if he completes the prayer, that is permissible and not disliked.
Reference: Zād al-Mustaqniʿ — Kashshāf al-Qināʿ — Maṭālib Ūlī al-Nuhā