It is narrated from al-Sha'bi that Sayyiduna Ali (may Allah be pleased with him) said regarding the son of a woman who underwent li'an, who left behind his brother and his mother: For the mother is a third and for the brother a sixth, and the remainder is returned to the two of them in proportion to what they inherited. Abdullah (may Allah be pleased with him) said: For the brother is a sixth, and the rest is for the mother, for she is his 'asabah. Zayd (may Allah be pleased with him) said: For the mother is a third, for the brother a sixth, and the rest goes to the public treasury (bayt al-mal).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12493