Hadith: Whoever intentionally kills a believer shall be handed to the guardians of the killed person: if they will, they may kill him, and if they will, they may accept the blood money. [The blood money] is thirty 'Hiqqah', thirty 'Jadha‘ah', and forty 'Khalifah'. And whatever they agree upon shall be given to them

عن عمرو بن شعيب، عن أبيه، عن جده، أن رسول الله صلى الله عليه وسلم قال: «مَنْ قَتَل مؤمِنا متعمِّدًا دُفِعَ إلى أولياء المَقْتول، فإنْ شاءوا قَتَلوا، وإنْ شاءوا أَخَذوا الدِّيَة، وهي ثلاثون حِقَّة، وثلاثون جَذَعَة، وأَرْبعون خَلِفَة، وما صالحوا عليه فهو لهم، وذلك لتَشْدِيد العَقْل».
[حسن] - [رواه أبو داود والترمذي وابن ماجه وأحمد]

‘Amr ibn Shu‘ayb reported from his father, from his grandfather, that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Whoever intentionally kills a believer shall be handed to the guardians of the killed person: if they will, they may kill him, and if they will, they may accept the blood money. [The blood money] is thirty 'Hiqqah', thirty 'Jadha‘ah', and forty 'Khalifah'. And whatever they agree upon shall be given to them. That is because the blood money [in this case] should be exorbitant."
Hasan/Sound. - [Ibn Maajah]

Explanation

This Hadīth shows the ruling subsequent to killing a believer intentionally. The guardians of the killed person, from among his heirs, have two options. They may ask for legal retribution, so the judge kills him; a punishment that resembles his crime. Otherwise, they can accept the blood money that is mentioned in the Hadīth: thirty 'Hiqqah' she-camels ('Hiqqah' is a she-camel in its fourth year of age), thirty 'Jadha‘ah' she-camels ('Jadha‘ah' is a she-camel in its fifth year of age), and forty pregnant she-camels. The guardians of the killed person may also accept an extra amount that they agree upon with the payers of the blood money. This is because the blood money for intentional killing is aggravated due to the perpetrator's intent and determination to kill the victim.