He who introduced some good practice in Islam which was followed after him (by people) he would be assured of reward like one who followed it, without their rewards being diminished in any respect. And he who introduced some evil practice in Islam which had been followed subsequently (by others), he would be required to bear the burden like that of one who followed this (evil practice) without their's being diminished in any respect.
جَاءَ نَاسٌ مِنَ الأَعْرَابِ إِلَى رَسُولِ اللَّهِ ﷺ عَلَيْهِمُ الصُّوفُ فَرَأَى سُوءَ حَالِهِمْ قَدْ أَصَابَتْهُمْ حَاجَةٌ فَحَثَّ النَّاسَ عَلَى الصَّدَقَةِ فَأَبْطَئُوا عَنْهُ حَتَّى رُئِيَ ذَلِكَ فِي وَجْهِهِ قَالَ ثُمَّ إِنَّ رَجُلاً مِنَ الأَنْصَارِ جَاءَ بِصُرَّةٍ مِنْ وَرِقٍ ثُمَّ جَاءَ آخَرُ ثُمَّ تَتَابَعُوا حَتَّى عُرِفَ السُّرُورُ فِي وَجْهِهِ فَقَالَ رَسُولُ اللَّهِ ﷺ مَنْ سَنَّ فِي الإِسْلاَمِ سُنَّةً حَسَنَةً فَعُمِلَ بِهَا بَعْدَهُ كُتِبَ لَهُ مِثْلُ أَجْرِ مَنْ عَمِلَ بِهَا وَلاَ يَنْقُصُ مِنْ أُجُورِهِمْ شَىْءٌ وَمَنْ سَنَّ فِي الإِسْلاَمِ سُنَّةً سَيِّئَةً فَعُمِلَ بِهَا بَعْدَهُ كُتِبَ عَلَيْهِ مِثْلُ وِزْرِ مَنْ عَمِلَ بِهَا وَلاَ يَنْقُصُ مِنْ أَوْزَارِهِمْ شَىْءٌ