The Messenger of Allah ﷺ said, "Whoever attaches himself as a client to a people without the permission of his own patrons, upon him is the curse of Allah and the angels. No obligatory act nor voluntary act will be accepted from him."
أن رسول الله ﷺ قال: من تولى قوما بغير إذن مواليه فعليه لعنة الله والملائكة، لا يقبل منه عدل، ولا صرف.
Muslim narrated it in "Manumission" (1508:18) from Qutayba bin Saeed, who said, Yaqub bin Abdul-Rahman Qari narrated to us, from Suhayl, from his father, from Abu Hurairah, and he mentioned it.