Sahih Muslim
Sahih Muslim Book 009, Hadith Number 3599.
Hadith Section/Book: Sahih Muslim Sahih Muslim Book 09. Divorce
Chapter: It is forbidden for a slave to take anyone as his ally except one who emancipates him.
Abu Huraira (Allah be pleased with him) reported that Allah's Messenger (may peace be upon him) said: He who takes anyone as his ally without the consent of his previous master, there will be the curse of Allah and that of His angels upon him, and neither, any obligatory act of his nor the supererogatory one will be accepted (by Allah).