Musannaf Ibn Abi Shayba 18:714

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Ash'ath, from al-Sha'bi, and from al-Hakam, from Ibrahim, who said: The first to impose the *'ata* (stipend) was Umar ibn al-Khattab. He imposed within it the full blood money (diyah) over three years, two-thirds of the blood money over two years, half of it over two years, and a third of it over one year. And whatever is less than that is in one year.