Musannaf Ibn Abi Shayba 26:264

Waki’ narrated to us, from Fitr, from a sheikh from the people of Medina, from Kharijah ibn Zayd, from Zayd ibn Thabit, who said: If the grandmother from the mother’s side is closer than the grandmother from the father’s side, she has the sixth. And if the grandmother from the mother’s side is [equally] as close as the grandmother from the mother’s side, the sixth is between them.