Saiduna Umar bin Abdul Aziz (may Allah have mercy on him) said: "If the Romans take a Muslim captive, then it is forbidden for the Muslims to return him to disbelief; rather, they should pay ransom for him with whatever is possible. Allah the Exalted has said: «وَإِنْيَأْتُوكُمْأُسَارَىتُفَادُوهُمْ»."
Hadith Referenceسنن سعید بن منصور / كتاب الجهاد / 3995