Sunan Ibn Majah, Book 1, Hadith 215 (In-Book) ⓘ

Kathir bin 'Abdullah narrated from his father, that his grandfather said: "I heard the Messenger of Allah say: 'Whoever revives a Sunnah of mine that dies out after I am gone, he will have a reward equivalent to that of those among the people who act upon it, without that detracting from their reward in the slightest. Whoever introduces an innovation (Bid'ah) with which Allah and his Messenger are not pleased, he will have a (burden of) sin equivalent to that of those among the people who act upon it, without that detracting from their sins in the slightest.'"

حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى، حَدَّثَنَا إِسْمَاعِيلُ بْنُ أَبِي أُوَيْسٍ، حَدَّثَنِي كَثِيرُ بْنُ عَبْدِ اللَّهِ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ يَقُولُ ‏ "‏ مَنْ أَحْيَا سُنَّةً مِنْ سُنَّتِي قَدْ أُمِيتَتْ بَعْدِي فَإِنَّ لَهُ مِنَ الأَجْرِ مِثْلَ أَجْرِ مَنْ عَمِلَ بِهَا مِنَ النَّاسِ لاَ يَنْقُصُ مِنْ أُجُورِ النَّاسِ شَيْئًا وَمَنِ ابْتَدَعَ بِدْعَةً لاَ يَرْضَاهَا اللَّهُ وَرَسُولُهُ فَإِنَّ عَلَيْهِ مِثْلَ إِثْمِ مَنْ عَمِلَ بِهَا مِنَ النَّاسِ لاَ يَنْقُصُ مِنْ آثَامِ النَّاسِ شَيْئًا ‏" ‏ ‏.‏