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.'" (FR)
English reference (FR): Vol. 1, Book 1, Hadith 203 (FR)
Arabic reference (FR): Book 1, Hadith 208 (FR)