Sayyiduna Anas bin Malik (may Allah be pleased with him) says that some men of the Ansar sought permission from the Messenger of Allah (peace and blessings be upon him) and said: Permit us to waive the ransom of our nephew (sister's son) Abbas (may Allah be pleased with him). He (peace and blessings be upon him) said: "By Allah! You shall not leave out even a single dirham."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18043