What is innovation in islam

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There are lots of Muslims today who greet JUMMAH MUBARAK on Friday.
I know that its an innovation as neither sahabi nor the Prophet Muhammad (SAW) greeted or said that.
I come across many muslims who argue about it being valid and saying its nothing wrong. I have read that all innovations in Islam are forbidden.
Please help me with a hadith or any reference of Islamic scholar that states JUMMAH MUBARAK being invalid.
Thanks.