Sayyiduna Imran bin Husayn (may Allah be pleased with him) said: In the time of Ignorance (Jahiliyyah), a man from the tribe of Hudhayl killed a man from the tribe of Khuza'ah, and that Hudhali remained in hiding. Then when the day of the Conquest of Makkah came and the announcement was made, he came out into the open. A man from Khuza'ah met him and slaughtered him as a sheep is slaughtered. The matter reached the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Did he kill him before the announcement or after the announcement?" People said: "He killed him after the announcement." The Prophet (peace and blessings be upon him) said: "If I were to kill a believer in retaliation for a disbeliever, I would have killed him for him; but pay his blood money." So they paid his blood money, and it was the first blood money in Islam.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15300