It is narrated from Imam al-Sha'bi (may Allah have mercy on him) that Sayyiduna Umar and Sayyiduna Abdullah (may Allah be pleased with them both) said, regarding a mother, a husband, maternal half-brothers and full brothers: For the husband is a half and for the mother a sixth; and they both made the maternal half-brothers and the full brothers share in the third, their males and females being equal in it. And they said: The father has only increased them in closeness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12475