Musannaf Ibn Abi Shaybah

Hazrat Qasim narrates that a man passed away and he left behind his two grandmothers, that is, his maternal and paternal grandmothers. Hazrat Abu Bakr (may Allah be pleased with him) made the maternal grandmother an heir and deprived the other. Then an Ansari said that if these two grandmothers had died and their sons were alive, then the son of the grandmother whom you made an heir would not inherit, and the son of the one whom you left out would inherit. Thus, Hazrat Abu Bakr (may Allah be pleased with him) also made her an heir and included them both in the one-sixth share of the wealth.