And he said: Whoever enters into a contract of allegiance with any people without the permission of his masters, upon him is the curse of Allah and all the angels. Allah will not accept any obligatory or voluntary act from him.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9400
Hadith Gradingحکم دارالسلام:إسنادہ صحیح ، م : 1508