Musannaf Ibn Abi Shayba 26:197

Ibn Fudayl narrated to us, from Bassam, from Fudayl, from Ibrahim, who said: Abdullah and Zayd used to assign one-third to the grandfather and two-thirds to the siblings. Regarding a man who left four brothers (full) and two sisters (full) and a grandfather, he said: Ali used to divide it into sixths; the sixth was for him (the grandfather). Ali would not assign the grandfather less than a sixth with the siblings, and what remained was for the male the portion of two females. Abdullah and Zayd used to give the grandfather one-third and the siblings two-thirds, for the male the portion of two females. And regarding five brothers and a grandfather, he said: For the grandfather, according to the opinion of Ali, is the sixth, and for the siblings is five-sixths. Abdullah and Zayd used to give the grandfather one-third and the siblings two-thirds.