Abu Huraira (Allah be pleased with him) reported Allah's Messenger (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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حُسَيْنُ بْنُ عَلِيٍّ الْجُعْفِيُّ، عَنْ زَائِدَةَ، عَنْ سُلَيْمَانَ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " الْمَدِينَةُ حَرَمٌ فَمَنْ أَحْدَثَ فِيهَا حَدَثًا أَوْ آوَى مُحْدِثًا فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ وَالنَّاسِ أَجْمَعِينَ لاَ يُقْبَلُ مِنْهُ يَوْمَ الْقِيَامَةِ عَدْلٌ وَلاَ صَرْفٌ " .
sunnah.com