Musannaf Ibn Abi Shayba 26:136

Waki’ narrated to us, he said: Sufyan narrated to us, from Mansur, from Ibrahim, from Masruq, who said: Abdullah was approached regarding a mother and maternal siblings. He gave the mother one-sixth and the siblings one-third, and he returned what remained to the mother. He said: "The mother is the *‘asabah* (residuary heir) of the one who has no *‘asabah*." Ibn Mas’ud would not return [the remainder] to a paternal sister along with a full sister, nor to a granddaughter along with a daughter of the loins.