Imām al-Shātibi al-Andalusi said, “Derived from the statement of the Mubtadi’ [innovator] is [their understanding that]: the Shari’ah is incomplete. For if they truly believed in the completion and perfection of the Shari’ah – they would not have resorted to innovation.”
Ibn al-Mājīshün said, “I heard Malik [b. Anas] say, ‘Whoever introduces an innovation (bid’ah) into Islam that he views as good has alleged that Muhammad ﷺ betrayed Messengership. Allaah ﷻ said, “This day, I have perfected your religion for you.. (5:3) [from this we can understand that] whatever was not part of the religion that day is not part of the religion today.'”
→ al-I’tiṣām 1/62 | Imām Abu Isḥāq al-Shātibi [Edition: at-Tawhid] | Translated by: Abu Dawūd [Mustafa b. Saalih] al-Hushayshi
Note [Abu Dawud al-Hushayshi]: to briefly explain the point about the implication of bid’ah being allegations against the Messenger’s conveyance of the Message – it means those who introduce new matters inadvertently disregard the Shari’ah being complete/perfect at the time of the Prophet ﷺ.