Abu Hurairah (RA) reported that Allah's Messenger (ﷺ) 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).
Abu Hurairah (RA) 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 supererogatory acts on the Day of Resurrection.