Sayyiduna al-Hakam bin Umayr al-Thumali (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "The most grievous matter, the heaviest burden, and the evil that never ends is the open display of innovations (bid'ah)."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3181