How should the missed four-unit prayers be made up when one is a traveler (musafir)?

If four-unit (rak‘ah) prayers are missed while traveling, they are made up as two units, whether the make-up (qada) is performed during the journey or after returning home. However, if a prayer that was missed while not traveling is made up during a journey, it is performed as four units (Merghinani, al-Hidaya, 1/81). According to the Shafi‘i school, a prayer missed during travel is made up as four units when one is resident (Shirbini, Mughni’l-muhtaj, 1/516).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu