From Sahih Muslim (Hadith 3791):
Abu Huraira (Allah be pleased with him) 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)
Reference: Muslim 3791