Sunan an-Nasai 18378
In-book no 3642أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ لَوْ غَضَّ النَّاسُ إِلَى الرُّبُعِ لأَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " الثُّلُثَ وَالثُّلُثُ كَثِيرٌ أَوْ كَبِيرٌ ".
It was narrated that Ibn 'Abbas said: "If the people were to reduce (their bequests) to one-quarter (of their wealth, that would be better), because the Messenger of Allah said: 'One-third, and one-third is much or large.'"