It is narrated from al-Qasim bin Muhammad that two grandmothers came to Sayyiduna Abu Bakr (may Allah be pleased with him), a maternal grandmother and a paternal grandmother. Sayyiduna Abu Bakr (may Allah be pleased with him) gave the inheritance to the maternal grandmother but did not give it to the paternal grandmother. Abd al-Rahman bin Sahl (may Allah be pleased with him) said: O Successor of the Messenger of Allah (peace and blessings be upon him)! You have given the inheritance to the one who, if she were to die, he (the deceased) would not be her heir. So Sayyiduna Abu Bakr (may Allah be pleased with him) assigned the sixth to the two of them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12343