In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
The Prophet [sallal laahu alayhi wasallam] said:
وشر الأمور محدثاتها
”The worst of all affairs are the newly invented matters [in the religion].”
He [sallal-laahu-alayhi-wasallam] said this whilst there were no Mubtadi’ [innovators in the religion] to be found. He warned against it because Allaah informed him that many (people) of this Ummah will be misguided, split (into deviated sects) and follow the (misguided) ways of those who came before them [i.e. the Yahood and Nasaaraa].
Allaah informed him [sallal-laahu-alayhi-wasallam] and he conveyed these realities to the Ummah. So due to his [sallal-laahu-alayhi-wasallam] enmity towards bidah, his hatred for it and due to fearing for his Ummah regarding it; he warned them against it in every or most of his khutbahs. He described it to be the worst of all affairs (in the religion):وشر الأمور محدثاتها -The worst of all affairs are the newly invented matters [in the religion]; and these newly invented matters [in the religion] can either be affairs of Kufr Akbar or Shirk Akbar, or Shirk Asghar; and they can be major sins and so on. They are the worst of all affairs [in the religion] and Allaah’s Refuge is sought.
[Source: الذريعة إلى بيان مقاصد كتاب الشريعة للإمام المحدث أبي بكر محمد بن الحسين الآجري – vol 1. Page: 219]