«وَوَرِثَهُ أَبَوَاهُ فَلِأُمِّهِ الثُّلُثُ»
“If his two parents inherit him, his mother receives one third.”
In this stated case the parents are the heirs under discussion. The mother's share is expressly assigned as one-third. The remaining two-thirds therefore belong to the father as the other heir.
Worked calculation: for an estate of 90 units, the mother receives 30 and the father 60. The father's amount is inferred without a separate numerical statement because of the complete two-person division.
This is not a universal assertion that a father always receives two-thirds. Other heirs may change the distribution. Necessity operates only within the stated legal circumstances.