Musannaf Ibn Abi Shayba 26:254

Husayn ibn Ali narrated to us, from Za'idah, from Mansur, who said: Ibrahim said: The Prophet (peace and blessings of Allah be upon him) assigned one-sixth to the grandmother from the mother's side and the two grandmothers from the father's side. Za'idah said: I said to Mansur, "Are those from the father's side the mother of his father and the mother of his mother?" He said, "Yes."