Sahih Muslim 3791
صحيح مسلم#3791SahihAbu 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)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 20:22