Sahih al-Bukhari, Volume 4, Book 51, Hadith 6 (USC-MSA) ⓘ
Narrated Ibn `Abbas: I recommend that people reduce the proportion of what they bequeath by will to the fourth (of the whole legacy), for Allah's Messenger said, "One-third, yet even one third is too much."
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا سُفْيَانُ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ لَوْ غَضَّ النَّاسُ إِلَى الرُّبْعِ، لأَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " الثُّلُثُ، وَالثُّلُثُ كَثِيرٌ أَوْ كَبِيرٌ " .
also referenced under: Book 55, Hadith 6 (In-Book) & Hadith 2743 (DarusSalam)