Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: A man made a will for another man of "a share" of his wealth, and the Prophet (peace and blessings be upon him) determined that "share" to be "one sixth".
Al-Bazzar recorded it; its chain contains Muhammad bin Ubaydullah al-Arzami, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الوصايا / 7098