Musannaf Ibn Abi Shayba 26:263

Waki’ narrated to us, he said: Bashir narrated to us, from Abdullah ibn Dhakwan, from Kharijah ibn Zayd, he said: If the grandmother from the mother’s side is closer than the grandmother from the father’s side, the sixth is for her. And if the grandmother from the mother’s side and the grandmother from the father’s side are equal, the sixth is between them.