It is narrated from Sayyiduna Amr bin Awf (may Allah be pleased with him) that in the letter of the Prophet (peace and blessings be upon him) this was also written: "Every group shall ransom its captive with kindness and justice among the believers; and it is upon the believers that they do not leave anyone among them who is weighed down by debt without giving him (help) in ransom or blood money."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16370