Sunan Ibn Majah, Chapter: The Book of the Sunnah

from his father, that his grandfather said: "I heard the Messenger of Allah (saw) say: 'Whoever revives a Sunnah of mine that dies out after I am gone, he will have a reward equivalent to that of those among the people who act upon it, without that detracting from their reward in the slightest. Whoever introduces an innovation (Bidah) with which Allah and His Messenger are not pleased, he will have a (burden of) sin equivalent to that of those among the people who act upon it, without that detracting from their sins in the slightest.'" (Daif)