Sayyiduna Abu Juhayfah (may Allah be pleased with him) said: I asked Sayyiduna Ali (may Allah be pleased with him): Do you have anything of the revelation? He said: "No. By the One who splits the seed and creates living beings! I know nothing beyond the understanding of the Qur'an that Allah Almighty gives to His servant, and this scroll." I said: What is in this scroll? He said: "Blood money, freeing captives (from the enemy), and that a believer will not be killed in return for a polytheist."
Zuhayr said: I asked Mutarrif: What is 'fikak al-asir'? He said: That the captive be freed from the enemy; the Sunnah has been established on this. And Mutarrif further said that 'al-aql' means blood money.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15909