Allah, the Blessed and Exalted, says: "And if a man or a woman whose inheritance is in question is kalalah (that is, has neither parents nor children), and has a brother or a sister, then for each one of them is a sixth; but if they are more than that, they share in a third." [Surah al-Nisa: 12]
Sayyiduna Sa'd (may Allah be pleased with him) used to recite this verse: "And if a man or a woman whose inheritance is in question is kalalah and has a brother or a sister" [Surah al-Nisa: 12] from the mother's side.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12322