It is narrated from Alaa bin Ziyad that Ziyad bin Matar made a will stating, "My will is according to whatever the jurists of Basra agree upon." I asked them, and those jurists agreed upon making a will for one-fifth (of the wealth).
Hadith Referenceسنن دارمي / من كتاب الوصايا / 3229
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده صحيح إلى زياد بن مطر، [مكتبه الشامله نمبر: 3240]
Hadith Takhrijاس اثر کی سند صحیح ہے۔ دیکھئے: [سعيد بن منصور 336]
Brief Explanation
(Commentary on Hadith 3228)
The jurists of Basrah agreed upon the one-fifth (khums) because the Messenger of Allah sallallahu alayhi wa sallam said: «الثلث و الثلث كثير», meaning, you may bequeath one-third of your wealth, although even this is much. And in one narration it is mentioned: «الربع», meaning, should I bequeath one-fourth of my wealth? So the Prophet sallallahu alayhi wa sallam said: No.
However, this narration is not authentic, as will be mentioned.