Kharijah bin Zayd (may Allah have mercy on him) narrates from his father (may Allah be pleased with him), according to the explanation of Abu al-Zinad, that the mother's inheritance, when her son or daughter dies and leaves children or son's children, male or female, or leaves two brothers, male or female, whether full, paternal or uterine, is a sixth for the mother. If the deceased leaves none of the above relatives, the mother has a full third, except in only two cases. These two are: a man dies leaving his wife and his parents, so his wife has a quarter and his mother has a third of the remainder, which is a quarter of the whole estate; and a woman dies leaving her husband and her parents, so her husband has a half and her mother has a third of the remainder, which is a sixth of the whole estate.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12294