Here is a detailed explanation of how deviation entered into Islam. It has been extracted from a lesson in the series explaining Usool-us-Sittah for Islam Tees by Abu Iyaad Amjad Rafiq (hafidhahullah Ta'ala).
It has been digitally edited before being posted below, walhamdulillah.
Abu Hurairah (رضي الله عنه) narrated that the Prophet ﷺ said: "Whoever invites to guidance will be rewarded like those who follow him in the guidance are rewarded, and that will not decrease the reward of those who have followed him." [Collected by Imām Muslim]
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