It is reported from Anas bin Sirin that a man asked Ibn Umar (may Allah be pleased with them both) about a man who died leaving his mother and two brothers. He said: "Go to Zayd (may Allah be pleased with him) and ask him, then inform me too of what Zayd says." He went to Zayd (may Allah be pleased with him), and he said: "The mother is blocked from the third; she has a sixth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12296