Yahya bin Saeed reported,

Muawiya bin Abu Sufyan wrote to Zayd bin Thabit asking him about the grandfather's share of inheritance, and Zayd bin Thabit wrote back to him, "You have written to me asking about the grandfather, and Allah knows best. That is a matter in which no one used to give judgment except the commanders," meaning the caliphs. "I was present with the two caliphs before you, and they would give him half along with a single brother, and a third along with two. If the brothers were many, they would not reduce him below a third."

Malik 1121

Sahih

حدثني يحيى عن مالك

عن يحيى بن سعيد أنه بلغه

أن معاوية بن أبي سفيان كتب إلى زيد بن ثابت يسأله عن الجد فكتب إليه زيد بن ثابت إنك كتبت إلي تسألني عن الجد والله أعلم وذلك مما لم يكن يقضي فيه إلا الأمراء يعني الخلفاء وقد حضرت الخليفتين قبلك يعطيانه النصف مع الأخ الواحد والثلث مع الاثنين فإن كثرت الإخوة لم ينقصوه من الثلث

مالك 1121صحيح
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Reference
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Ayat
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Muwatta Malik
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Obligatory duties
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Laithi 1121
Malik 1121