Ibn Abbas said, "If only people would reduce from a third to a fourth, for the Messenger of Allah ﷺ said, 'A third, and a third is a lot.'"
لو أن الناس غضوا من الثلث إلى الربع؛ فإن رسول الله ﷺ قال: الثلث، والثلث كثير.
Bukhari narrated it in the Book of Wills (2743), and Muslim in the Book of Wills (1629), both from the hadith of Hisham bin Urwa, from his father, from Ibn Abbas, who mentioned it.