Musannaf Ibn Abi Shayba 26:196

'Abd al-A'la narrated to us, from Yunus, from al-Hasan, that Zayd used to divide [the inheritance] with the grandfather along with one or two [brothers]. If they were three, he [the grandfather] would have one-third of the entire estate. If there were fixed shares along with him, he would look at what was better for him: if the one-third was better for him, he would be given it, and if the division (muqasamah) was better for him, he would divide. And he would not be decreased below one-sixth of the entire estate.