Musannaf Ibn Abi Shayba 26:201

Ibn Fudayl narrated to us, from Bassam, from Fudayl, from Ibrahim, regarding a man who left behind his grandfather and his brother (from both his father and mother): The grandfather receives half, and his brother receives half, according to the opinion of Ali, Abdullah, and Zayd. They said regarding a man who left behind his grandfather and his siblings (from both his father and mother): The grandfather receives one-third, and the siblings receive two-thirds, according to all of their opinions. Abu Bakr said: These are among the shares. If there is a sister or a brother and a grandfather, the grandfather receives half, and the sister or brother receives half. If there are two brothers, the grandfather receives one-third, and the two brothers receive two-thirds.