It is reported from al-Hasan (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) "made three grandmothers heirs."
This too is mursal (disconnected), and it supports the first one; and it is what has been narrated from a group of the Companions of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12350