Kharijah bin Zayd (may Allah have mercy on him) narrates from his father that he used to rule that the mother is reduced (in her share) by two brothers. They said to him: "O Abu Sa'id! Allah Almighty says: 'But if he has brothers, then for his mother is a sixth' [Surah al-Nisa: 11], and you reduce her by two brothers?" Abu Sa'id (may Allah be pleased with him) said: "The Arabs call two brothers 'brothers'." They said: "O Abu Sa'id! Have you made a mistake? They are only eight pairs: of sheep two and two, of goats two and two, of camels two and two, and of cattle two and two." Abu Sa'id (may Allah be pleased with him) said: "No. Allah Almighty says: 'And made of him the two mates, the male and the female' [Surah al-Qiyamah: 39]. So they are two mates; each one of them is a mate." He says: The male is a mate and the female is a mate.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12295