Musannaf Ibn Abi Shayba 13:2899

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, from Ibn Sirin, who said: Shurayh said: "I do not permit the testimony of an adversary, nor a suspicious person, nor one who wards off a fine, nor a partner for his partner, nor a hired worker for the one who hired him, nor a slave for his master."