It is narrated from al-Sha'bi (may Allah have mercy on him) that Ali and Zayd (may Allah be pleased with them both) said: For the husband is a half, for the mother a sixth and for the maternal half-brothers a third. And they did not make the full brothers share with the maternal half-brothers, and they both said: They are 'asabah (residuary heirs); if anything remains it will be theirs, and if nothing remains they will get nothing.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12480