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).
Related hadith on this topic
Explanation & Benefits
Shaykh Maulana Abdul Aziz Alvi
Hadith Gloss:
Vocabulary of the Hadith:
‘Adl:
measurement,
uprightness,
recompense,
ransom,
guarantor,
only weight,
price,
blood money recommendation,
bribery.
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 3791