Musannaf Ibn Abi Shayba 26:306

Waki’ narrated to us, from Sufyan, from the one who heard al-Sha’bi, from Ali and Abdullah, that they both said regarding the son of a woman involved in *li’an* (imprecation) who died and left behind his mother and his maternal brother: Ali used to say: "The mother gets one-third, and the brother gets one-sixth, and what remains is returned to them, the two-thirds and the one-third." Ibn Mas’ud used to say: "The mother gets one-third, and the brother gets one-sixth, and what remains is returned to the mother." Abu Bakr said: "So this, according to all of their statements, results in a base of six."