It is narrated from Sayyiduna Abu Abd al-Rahman al-Makhzumi (may Allah be pleased with him) that Sa'd (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him) about bequests, and he (peace and blessings be upon him) said: "A bequest of a quarter (of the wealth) is permissible for him."
Hadith Referenceالمعجم الكبير للطبراني / مسند من يعرف بالكنى / 18951