Maliks Muwatta Book 27, Hadith Number 1.

Section : Inheritance of Grandfathers.

Yahya related to me from Malik from Yahya ibn Said that he had heard that Muawiya ibn Abi Sufyan wrote to Zayd ibn Thabit asking him about the grandfather. Zayd ibn Thabit wrote to him, “You have written to me asking me about the grandfather. Allah knows best. That is part of what is only determined by the amirs, i.e. the khalifs. I was present with two khalifs before you who gave the grandfather a half with one sibling, and a third with two. If there were more siblings, they did not decrease his third.”