It is narrated from Ibrahim (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) made three grandmothers heirs to the sixth. The narrator said: I said to Ibrahim (may Allah have mercy on him): Who were they? He said: Two from the father's side and one from the mother's side.
This is mursal (disconnected). It has also been narrated through Abd al-Rahman bin Yazid from the Prophet (peace and blessings be upon him), and that too is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12348