Ubayd bin Nadlah said: Umar and Abdullah (may Allah be pleased with them both) used to have the grandfather share with the brothers as long as a sixth was not better for him than sharing with them. Then Umar (may Allah be pleased with him) wrote to Abdullah: "I can only see that we have been unfair to the grandfather. When this letter of mine reaches you, have him share with the brothers as long as a third is not better for him than sharing with them." So Abdullah (may Allah be pleased with him) acted upon that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12437