Imam al-Bukhari mentioned in his Sahih:
Ibn ‘Umar رضي الله عنهما considered the Khawarij and the heretics (al-Mulhidin) as the worst of Allah’s creatures and said: “These people took some Verses that had been revealed concerning the disbelievers and interpreted them and applied them to the believers.”
[English Sahih al-Bukhari; Darussalam, Volume 9, Page 49-50, Chapter on Killing the Khawarij and the Mulhidin]
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وَكَانَ ابْنُ عُمَرَ يَرَاهُمْ شِرَارَ خَلْقِ اللَّهِ وَقَالَ إِنَّهُمُ انْطَلَقُوا إِلَى آيَاتٍ نَزَلَتْ فِي الْكُفَّارِ فَجَعَلُوهَا عَلَى الْمُؤْمِنِينَ.
[صحيح البخاري؛ كتاب استتابة المرتدين والمعاندين وقتالهم؛ باب قَتْلِ الْخَوَارِجِ وَالْمُلْحِدِينَ بَعْدَ إِقَامَةِ الْحُجَّةِ عَلَيْهِمْ]