Musannaf Ibn Abi Shayba 26:466

Al-Muharibi narrated to us, from al-A'mash, from Ibrahim, regarding brotherhood: If one of them claims a person is a brother and the others deny it, he said: "He enters with them in the status of a slave who is owned by brothers, and one of them emancipates his share." He said: "Amir, al-Hakam, and their companions used to say: 'He does not enter [into the brotherhood] except in the share of the one who acknowledged him.'"