Abdullah bin Salamah said that Sayyiduna Ali (may Allah be pleased with him) was asked about the maternal half-brothers, and he said: "What do you think, if they were a hundred, would you give them more than the third?" They said: "No." He said: "Then I will not reduce them from it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12483