It is narrated from Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) "judged regarding the inheritance of the paternal and maternal grandmother in this way: he placed the two of them equally in the sixth."
Ishaq's narration from Ubadah is mursal (disconnected).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12344