Musannaf Ibn Abi Shayba 26:187

Abu Mu'awiyah narrated to us, from al-A'mash, from Ibrahim, from 'Ubayd ibn Nadlah, who said: 'Umar and 'Abdullah used to divide [the inheritance] with the grandfather alongside the brothers, as long as the sixth was better for him than dividing with them. Then 'Umar wrote to 'Abdullah: "I do not see but that we have been unjust to the grandfather. So when this letter of mine reaches you, divide with him alongside the brothers, as long as the third is better for him than dividing with them." And 'Abdullah adopted that.