Muwatta Imam Malik Book of Fara'id, Hadith 1

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."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّهُ بَلَغَهُ أَنَّ مُعَاوِيَةَ بْنَ أَبِي سُفْيَانَ، كَتَبَ إِلَى زَيْدِ بْنِ ثَابِتٍ يَسْأَلُهُ عَنِ الْجَدِّ، فَكَتَبَ إِلَيْهِ زَيْدُ بْنُ ثَابِتٍ إِنَّكَ كَتَبْتَ إِلَىَّ تَسْأَلُنِي عَنِ الْجَدِّ، وَاللَّهُ، أَعْلَمُ وَذَلِكَ مِمَّا لَمْ يَكُنْ يَقْضِي فِيهِ إِلاَّ الأُمَرَاءُ - يَعْنِي الْخُلَفَاءَ - وَقَدْ حَضَرْتُ الْخَلِيفَتَيْنِ قَبْلَكَ يُعْطِيَانِهِ النِّصْفَ مَعَ الأَخِ الْوَاحِدِ وَالثُّلُثَ مَعَ الاِثْنَيْنِ فَإِنْ كَثُرَتِ الإِخْوَةُ لَمْ يُنَقِّصُوهُ مِنَ الثُّلُثِ .