Musnad Ahmad 28:241

Abu al-Nadr and Abd al-Samad narrated to us, they said: Muhammad narrated to us, Sulayman—meaning Ibn Musa—narrated to us, from Amr ibn Shu’ayb, from his father, from his grandfather, that the Prophet (peace and blessings of Allah be upon him) said: "Whoever kills intentionally is handed over to the guardians of the slain. If they wish, they may kill him, and if they wish, they may take the blood money (diyah). It is thirty hiqqah (four-year-old camels), thirty jadha’ah (five-year-old camels), and forty khalfah (pregnant camels). That is the blood money for intentional killing. Whatever they agree upon as a settlement, it is theirs, and that is the intensification of the blood money."