Hammad narrates from Ibrahim: Whoever among those who have a share (in the inheritance) pardons, his pardon is a valid pardon. Umar and Ibn Mas'ud (may Allah be pleased with them both) permitted the pardon of one of the heirs, and they did not ask whether it was a killing by treachery or otherwise.
Imam al-Shafi'i (may Allah have mercy on him) said: Some of our companions say concerning a man who kills another without any feud that the matter belongs to the imam, and he is not to wait for the heir of the slain; and they took as evidence the incident of Mujadhdhar bin Ziyad. If this narration were established, then it would be so; but until today I have not found that this narration is established, and returning the matter to the heir of the slain is the command of Allah. Allah Almighty says: "And whoever is killed wrongfully, We have given his heir authority." [Surah al-Isra: 33]
And He said: "But whoever is pardoned something by his brother, then let there be a following up with what is fair." [Surah al-Baqarah: 178]
The Shaykh said: What has reached us is that the story of Mujadhdhar bin Ziyad, which is from the narration of al-Waqidi, is munqati' (broken) and da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16060