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.
In-book : Book 19, Hadith 12
English translation : Book 18, Hadith 2890
19. 'Shares of Inheritance (Kitab Al-Fara''id)'
باب مَا جَاءَ فِي مِيرَاثِ الْجَدِّ