صحيح البخاري

Narrated Ibn `Abbas:

(During the early days of Islam), the inheritance used to be given to one's offspring and legacy used to be bequeathed to the parents, then Allah cancelled what He wished from that order and decreed that the male should be given the equivalent of the portion of two females, and for the parents one-sixth for each of them, and for one's wife one-eighth (if the deceased has children) and one-fourth (if he has no children), for one's husband one-half (if the deceased has no children) and one-fourth (if she has children).