Musannaf Ibn Abi Shayba 26:32

Ibn Numayr narrated to us, he said: Sufyan narrated to us, from Abd al-Rahman ibn al-Asbahani, from Ikrimah, who said: Ibn Abbas sent me to Zayd ibn Thabit to ask him about a husband and two parents. Zayd said: "For the husband is half, and for the mother is a third of what remains, which is the sixth." Ibn Abbas sent word to him: "Do you find this in the Book of Allah?" He said: "I dislike preferring a mother over a father." Ibn Abbas used to give the mother a third of the entire estate.