Musannaf Ibn Abi Shayba 26:231

Mu'awiyah ibn Hisham narrated to us, he said: Sufyan narrated to us, from al-A'mash, from Ibrahim, who said: Zayd used to include the grandfather with the brothers and sisters for a third. If it reached a third, he would give him the third, and the brothers and sisters would have what remained. Neither the brother through the mother nor the sister through the mother would have anything with the grandfather. The brother through the father would share with the brothers through the father and mother, but he would not inherit anything for them. If there was a brother through the father and mother and a grandfather, he would give the grandfather half. If there were two brothers and a grandfather, he would give him a third. If they increased, he would give him a third, and the brothers would have what remained. If there was a sister and a grandfather, he would give him two-thirds with the sister, and the sister would have a third. If there were two sisters, he would give them half, and he would have half, as long as sharing (muqasamah) was better for him. If the shares of a wife, a mother, or a husband were attached, he would give the people of the shares their shares, and with what remained, he would share with the brothers and sisters. If a third of what remained was better for him than sharing, he would give him a third of what remained. If sharing was better for him than a third of what remained, he would give him the sharing. If a sixth of the entire estate was better for him than sharing, he would give him the sixth. If sharing was better for him than a sixth of the entire estate, he would give him the sharing.