Musannaf Ibn Abi Shayba 26:156

Ibn Fudayl narrated to us, from Bassam, from Fudayl, who said: Ibrahim said regarding a Muslim woman who left behind a Muslim mother and she had brothers who were Christians, Jews, or disbelievers: Abdullah [ibn Mas'ud] judged that she [the mother] has one-sixth along with them, and he made them [the non-Muslim brothers] block [the mother from the remainder of the inheritance] while they do not inherit. The rest of the Companions of the Prophet, may Allah bless him and grant him peace, judged regarding her that they do not block [the mother] and they do not inherit. Abu Bakr [Ibn Abi Shayba] said: So, according to the judgment of the Companions of the Prophet, may Allah bless him and grant him peace, other than Abdullah, it is four shares, and it is for the *'asaba* (agnatic heirs). And according to the judgment of Abdullah, it is five shares, and it is for the *'asaba* by blood relation. Abu Bakr said: So this, according to all of their statements, is out of six shares. If it is according to the statement of Abdullah, then the mother has one-sixth, and five remain. And if it is according to the statement of the Companions of the Prophet, may Allah bless him and grant him peace, then the mother has one-third, which is two shares, and four are for the rest of the *'asaba*.