Imam al-Shafi'i (may Allah have mercy on him) says: We found the people of knowledge in general saying that the Messenger of Allah (peace and blessings be upon him) "fixed 100 camels upon the 'aqilah (paternal kin) for a free Muslim killed by mistake, and gave three years' respite for it, so that each year a third of the blood money is paid, in camels of specified ages."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16389
Amir al-Sha'bi (may Allah have mercy on him) says that Sayyiduna Umar (may Allah be pleased with him) fixed a term of three years for the full blood money, two years for two-thirds of the blood money, two years for half, and one year for a third of the blood money. And Imam Malik (may Allah have mercy on him) also said the same, (saying) that there are two years for the half because it exceeds a third.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16390
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