Abu Dawud Book 12, Hadith Number 2890.

Chapter : On the share of grandfather from inheritance.

Narated By Imran ibn Husayn : A man came to the Prophet (PBUH) and said: My son has died; what do I receive from his estate? He replied: You receive a sixth. When he turned away he called him and said: You receive another sixth. When he turned away, he called him and said: The other sixth is an allowance (beyond what is due).

Qatadah said: They (the Companions) did not know the heirs with whom he was given (a sixth).

Qatadah said: The minimum share given to the grandfather was a sixth.