Musannaf Ibn Abi Shayba 26:216

Waki’ narrated to us, from Sufyan, from Mansur, from Ibrahim, regarding ‘Umar concerning a sister, a mother, and a grandfather: He said: "The sister gets one-half, the mother gets one-sixth, and whatever remains is for the grandfather." Abu Bakr said: This, according to the view of ‘Ali and ‘Abdullah, is out of six shares. And according to the view of Zayd ibn Thabit, it is out of nine shares.