Musannaf Ibn Abi Shayba 26:199

Narrated to us Abd al-A’la, from Dawud, from Shahr ibn Hawshab, from Abd al-Rahman ibn Ghanam, who said: The first grandfather to inherit in Islam was Umar ibn al-Khattab. He wanted to choose the wealth [for himself], so I said to him, "O Commander of the Faithful, they are a tree beneath you," meaning his sons. Abu Bakr said: This is according to the opinion of Umar, Abdullah [ibn Mas’ud], and Zayd, [calculated] from three shares; the grandfather gets one-third, and what remains is for the brothers. And according to the opinion of Ali, [it is calculated] from six shares; the grandfather gets one-sixth, one share, and the brothers get five shares.