Sunan Ibn Majah, Book 1, Hadith 212 (In-Book) ⓘ

It was narrated that Abu Juhaifah said: "The Messenger of Allah said: 'Whoever introduces a good practice that is followed after him, will have a reward for that and the equivalent of their reward, without that detracting from their reward in the slightest. Whoever introduces an evil practice that is followed after him, will bear the burden of sin for that and the equivalent of their burden of sin, without that detracting from their burden in the slightest.'"

حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى، حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا أَبُو إِسْرَائِيلَ، عَنِ الْحَكَمِ، عَنْ أَبِي جُحَيْفَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ مَنْ سَنَّ سُنَّةً حَسَنَةً فَعُمِلَ بِهَا بَعْدَهُ كَانَ لَهُ أَجْرُهَا وَمِثْلُ أُجُورِهِمْ مِنْ غَيْرِ أَنْ يَنْقُصَ مِنْ أُجُورِهِمْ شَيْئًا وَمَنْ سَنَّ سُنَّةً سَيِّئَةً فَعُمِلَ بِهَا بَعْدَهُ كَانَ عَلَيْهِ وِزْرُهُ وَمِثْلُ أَوْزَارِهِمْ مِنْ غَيْرِ أَنْ يَنْقُصَ مِنْ أَوْزَارِهِمْ شَيْئًا ‏" ‏ ‏.‏