Yahya related to me from Malik from Yahya ibn Said that he hadheard that Muawiya ibn Abi Sufyan wrote to Zayd ibn Thabit asking himabout the grandfather. Zayd ibn Thabit wrote to him, "You have writtento me asking me about the grandfather. Allah knows best. That is partof what is only determined by the amirs, i.e. the khalifs. I waspresent with two khalifs before you who gave the grandfather a halfwith one sibling, and a third with two. If there were more siblings,they did not decrease his third."
USC-MSA web (English) reference: Book 27, Hadith 1
Arabic reference: Book 27, Hadith 1077