Musannaf Ibn Abi Shayba 26:256

Ibn Fudayl narrated to us, from Bassam, from Fudayl, who said: Ibrahim said: The grandmothers inherit one-sixth. If there is one, or two, or three, then the share is between them, according to the statement of Ali and Zayd. If three grandmothers gather, they are equal in relation to the deceased. He said: The share is between them equally, whether it is the mother’s grandmother, the father’s mother, or the mother’s mother. And according to the statement of Abdullah, if three grandmothers gather, the one-sixth is between them, even if some of them are closer in lineage, provided that some of them are not the mothers of others.