The evidence for this is what Abu Dawud narrated from Abdullah ibn Mas'ud, may Allah be pleased with him, from the Prophet, peace and blessings be upon him: that he mentioned the blood-money (1) in a manner similar to that described by the author.
There is another sunnah concerning this matter: the blood-money is due in thirds—thirty hiqqahs, thirty jadha'ahs, and forty khalifahs—that is, pregnant she-camels carrying offspring in their wombs (2).
Authentic hadiths have been reported for both. Some scholars have adopted this view, while others have adopted the other. The latter is more severe than the former. If it is asked, “Which of the two views should we follow?”
The answer is that this may be referred to the judgment of the Islamic ruler: if he considers it appropriate to prescribe it in this manner, he may do so; and if he considers it appropriate to prescribe it in the other manner, he may do so, according to what the circumstances require. As for the madhhab, the four specified age categories are binding in deliberate and شبه-deliberate killing, and in accidental killing with respect to the age categories—
(1) Reported by Ahmad (1/450); Abu Dawud in al-Diyat, chapter: “How much is the diyah (blood money)?” (4545); al-Nasai in al-Qasamah, chapter: “Mention of the ages of the camels prescribed for unintentional-homicide diyah” (8/43); al-Tirmidhi in al-Diyat, chapter: “What has been narrated regarding the number of camels in the diyah” (1386); and Ibn Majah in al-Diyat, chapter: “The diyah for unintentional homicide” (2631).
Al-Tirmidhi said: “We do not know it as a marfu report except through this route, though it has been narrated from Abdullah as a mawquf report.” Al-Daraqutni deemed it weak on several grounds. See al-Sunan (3364), al-Risalah edition, where he authenticated the mawquf report; Nasb al-Rayah (4/357); Khulasat al-Badr al-Munir (2235); and al-Talkhis (1695).
(2) Reported by al-Tirmidhi in al-Diyat, chapter: “What has been narrated regarding the number of camels in the diyah” (1387), and Ibn Majah in al-Diyat, chapter: “One who kills deliberately and they agree to accept the diyah” (2626), on the authority of Amr ibn Shuayb, from his father, from his grandfather. Al-Tirmidhi said: “A sound, uncommon hadith.”