Yazid bin Abi Habib says that Sayyiduna Ali (may Allah be pleased with him) fixed three years for the blood money in unintentional killing; and Yahya bin Sa'id says that the Sunnah is that respite of three years be given for the blood money.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16391