Sunnah (tradition) and Bid’ah (innovation) are two widely used concepts in various fields of Islamic sciences, with numerous rulings regarding them in beliefs and jurisprudence. Most attention around this issue has been focused on its instances, and less on its theoretical foundations and with a nature-oriented approach. In this article, by referring to narrations from the Ahl al-Bayt (a.s.) on the subject of bid’ah, the theoretical dimensions of sunnah and bid’ah such as: nature, typology, types, origin, formation process, and consequences of creating sunnah and bid’ah have been examined. In the narrations of the Ahl al-Bayt (a.s.), sunnah is divided into two types: religious and customary, and good and bad sunnahs fall under the category of customary sunnah. In contrast, bid’ah is also divided into religious bid’ah and customary bid’ah, where all instances of religious bid’ah are blameworthy, and the commendable instances of customary bid’ah are recognized as good customary sunnahs. The result of this research is that the infallible Imams (a.s.) have shown special attention to sunnah-centrism and avoidance of religious innovations, and have called any innovation in religion a misguidance. However, to encourage Muslims to create customary sunnahs with social extension and in line with religious teachings, they have considered numerous rewards, and in contrast, have predicted the sin of those who act upon the disliked customary sunnahs for the founder of that bad sunnah.