What must a Muslim do if he accidentally kills another Muslim, such as unintentionally killing him in a car accident?

In the case of accidental killing, two things are obligatory: blood money (Diyyah) and expiation (Kaffarah (definition)). The blood money is incumbent upon the killer's 'aqilah, who are his male relatives on his father's side. The ruler divides it among them according to kinship and wealth. The blood money being on the 'aqilah applies to accidental killing or semi-intentional killing, but not purely intentional killing. As for expiation, it is incumbent upon the killer, and it is the freeing of a believing slave. If one cannot find [a slave to free], then it is fasting for two consecutive months.