From Sahih Muslim (Hadith 3330):
Abu Huraira (Allah be pleased with him) reported Allah's Apostle (ﷺ) as saying:Medina is a sacred territory, so he who made any innovation in it. or gave protection to an innovator, there is upon him the curse of Allah, that of the angels and that of all the people. There would not be accepted on the Day of Resurrection either obligatory acts or supererogatory acts from him
Reference: Muslim 3330