Sunan Ibn Majah, Book 1, Hadith 208 (In-Book) ⓘ

It was narrated from Mundhir bin Jarir that his father said: "The Messenger of Allah said: 'Whoever introduces a good practice that is followed, he will receive its reward and a reward equivalent to that of those who follow it, without that detracting from their reward in their slightest. And whoever introduces a bad practice that is followed, he will receive its sin and a burden of sin equivalent to that of those who follow it, without that detracting from their burden in the slightest.'"

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الْمَلِكِ بْنِ أَبِي الشَّوَارِبِ، حَدَّثَنَا أَبُو عَوَانَةَ، حَدَّثَنَا عَبْدُ الْمَلِكِ بْنُ عُمَيْرٍ، عَنِ الْمُنْذِرِ بْنِ جَرِيرٍ، عَنْ أَبِيهِ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ مَنْ سَنَّ سُنَّةً حَسَنَةً فَعُمِلَ بِهَا كَانَ لَهُ أَجْرُهَا وَمِثْلُ أَجْرِ مَنْ عَمِلَ بِهَا لاَ يَنْقُصُ مِنْ أُجُورِهِمْ شَيْئًا وَمَنْ سَنَّ سُنَّةً سَيِّئَةً فَعُمِلَ بِهَا كَانَ عَلَيْهِ وِزْرُهَا وَوِزْرُ مَنْ عَمِلَ بِهَا مِنْ بَعْدِهِ لاَ يَنْقُصُ مِنْ أَوْزَارِهِمْ شَيْئًا ‏" ‏ ‏.‏