Musannaf Ibn Abi Shayba 26:36

'Abdah narrated to us, from al-A'mash, that Ibn 'Abbas sent to Zayd to ask him about a husband and two parents. Zayd said: "For the husband is half, and for the mother is one-third of what remains." Ibn 'Abbas said: "Do you find for her in the Book of Allah one-third of what remains?" Zayd said: "This is my opinion, and Allah knows best." Abu Bakr said: "These are six shares: for the husband three, for the mother one share, and for the father two shares."