It is narrated from Ibrahim (may Allah have mercy on him) that Sayyiduna Umar, Abdullah and Zayd (may Allah be pleased with them) said: For the husband is a half, and for the mother a sixth; and they made the full brothers and the maternal half-brothers share in the third, and they said: The father has only increased them in closeness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12474