Musannaf Ibn Abi Shayba 26:157

Ibn Fudayl narrated to us, from Bassam, from Fudayl, who said: Ibrahim said regarding a woman who left behind her husband, her maternal half-siblings who are free, and a son who is a slave: For her husband is the half, three shares; and for her maternal half-siblings is the third, two shares; and the sixth remains, so it is for the agnatic heirs (asaba), and her son who is a slave inherits nothing, according to the judgment of Ali. And Abdullah judged regarding it that for her husband is the quarter, one share and a half; and that her son blocks the maternal half-siblings if he is a slave, and her son inherits nothing, and he blocks the husband; and that the remaining three-quarters are for the agnatic heirs. And Zayd judged regarding it that for her husband is the half, three shares; and for her maternal half-siblings is the third, two shares; and what remains is in the public treasury (bayt al-mal) if there is no wala' (patronage) or kinship. Abu Bakr said: So this, according to the opinion of Ali and Zayd, is out of six shares, and according to the opinion of Abdullah ibn Mas'ud, it is out of four shares.