Kathir bin 'Abdullah bin 'Amr bin 'Awf Al-Muzani said:"My father told me narrating from my grandfather, that the Messenger of Allah said: 'Whoever revives a Sunnah of mine, which people then act upon, will have a reward equivalent to that of those who act upon it, without that detracting from their reward in the slightest. And whoever introduces an innovation (Bid'ah) that is acted upon, will have a burden of sins equivalent to that of those who act upon it, withot that detracting from the burden of those who act upon it in the slightest

English translation · Sunan Ibn Majah