Musannaf Ibn Abi Shayba 26:261

Ya'la narrated to us, from Yahya, from al-Qasim, who said: A man died and left behind his two grandmothers: his mother's mother and his father's mother. Abu Bakr inherited the mother's mother and left the other. A man from the Ansar said to him: "You have left a woman who, if the two grandmothers had died while their son was alive, would not have inherited anything from the one you inherited from; and you have left the one who would have inherited from her son's son." So Abu Bakr inherited her, and he made them both share in the one-sixth.