Sayyiduna Imran bin Husayn (may Allah be pleased with him) said that a man came to the Messenger of Allah (peace and blessings be upon him) and said: My son's son has died; what inheritance is there for me? He (peace and blessings be upon him) said: "You get a sixth." When he turned to leave, he (peace and blessings be upon him) called him back and said: "You get another sixth." When he turned to leave, he (peace and blessings be upon him) called him back and said: "The other sixth is a provision (tu'mah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12408