Amr bin Kharijah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) addressed us at Mina while he was on his mount. He (peace and blessings be upon him) said: "Allah Almighty has allotted to every person his share of the inheritance, so a bequest is not permissible for an heir."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12539