Al-Qasim bin Muhammad said: Two grandmothers came to Sayyiduna Abu Bakr (may Allah be pleased with him). Sayyiduna Abu Bakr (may Allah be pleased with him) intended to assign the sixth to the maternal grandmother. A man from the Ansar said: "Are you leaving out the one who, if she had died while the deceased was alive, he would have been her heir?" So Sayyiduna Abu Bakr (may Allah be pleased with him) gave the sixth to the two of them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12342