From Sahih Muslim (Hadith 3793):
Abu Huraira (Allah be pleased with him) reported Allah's Apostle (ﷺ) as saying:He who took the freed slave as his ally without the consent of his previous master, there is upon him the curse of Allah and that of His angels and that of the whole mankind, and there will not be accepted from him his obligatory acts or supercrogatory acts on the Day of Resurrection. This hadith is narrated through the same chain of transmitters, but with a slight change of words
Reference: Muslim 3793