Musannaf Ibn Abi Shayba 26:182

Yazid ibn Harun narrated to us, he said: Hammam narrated to us, from Qatadah, from al-Hasan, from Imran ibn Husayn, that a man came to the Prophet, may Allah bless him and grant him peace, and said: "My son's son has died, so what do I have of his inheritance?" He said: "The sixth." When he turned away, he called him and said: "You have another sixth." When he turned away, he called him and said: "The other sixth is a gift (tu'mah)."