Ibn Buraydah (may Allah have mercy on him) narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) "gave the grandmother a sixth when there was no mother."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12290
Kharijah bin Zayd (may Allah have mercy on him) narrates from his father that the meanings and principles of these inheritance shares are reported from Zayd bin Thabit (may Allah be pleased with him), and their explanation is from Abu al-Zinad (may Allah have mercy on him). It states that the maternal grandmother is entitled to nothing in the presence of the mother, and in other cases a sixth is assigned to her as a fixed share; and the paternal grandmother will inherit nothing in the presence of the mother or the father, and in other cases a sixth is assigned to her as a fixed share.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12291