What is the responsibility regarding the ransom (fidyah) of captured Muslims if it is possible to ransom them?

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Ransoming captured Muslims is obligatory if possible. This is the position of 'Umar bin 'Abd al-'Aziz, Malik, and Ishaq. Evidence includes the Hadith of the Prophet, peace be upon him: 'Feed the hungry, visit the sick, and ransom the captive' ('At'imu al-ja'i', wa 'oodu al-mareed', wa fakku al-'aani'). Furthermore, the Prophet's written covenant between the Muhajirun and Ansar stipulated that they must ransom their captives with what is reasonable ('bi al-ma'roof'). The Prophet himself ransomed two Muslims for one captive from Banu 'Uqayl.