Sunan Ibn Majah, Chapter: The Book of the Sunnah

"The Messenger of Allah (saw) said: 'Whoever introduces a good practice that is followed after him, will have the 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.'" (Sahih)