Sunan Ibn Majah, Chapter: The Book of the Sunnah

that his father said: "The Messenger of Allah (saw) 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 the 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.'" (Sahih)