Musannaf Ibn Abi Shayba 13:2164

Abu Bakr narrated to us, he said: Ali ibn Mushir narrated to us, from al-Shaybani, from al-Sha’bi, who said: I asked him about the partition wall (khas) which the people of this house and the people of that house claim. He said: It belongs to the one whom it adjoins, and it adjoins both of them equally.