Allaah (The Most High) said:

فَلْيَحْذَرِ الَّذِينَ يُخَالِفُونَ عَنْ أَمْرِهِ أَن تُصِيبَهُمْ فِتْنَةٌ أَوْ يُصِيبَهُمْ عَذَابٌ أَلِيمٌ

And let those who oppose the Messenger’s (Muhammad) commandment (i.e. his Sunnah legal ways, orders, acts of worship, statements, etc.) beware, lest some Fitnah befall them or a painful torment be inflicted on them. [24:63]

Reminders from Imaam Ibnul Jawzi (rahimahullaah)

There are three statements of the scholars regarding the Fit’nah intended in this Ayah: Ibn Abbaas (radiyallaahu-anhumaa) said that this fitnah is ‘misguidance’; Mujaahid (rahimahullaah) said that it is ‘tribulation in the worldly life’; Muqaatil and As-Sadiy (rahimahumallaah) said that it is ‘disbelief’

And there are two statements of the scholars regarding the punishment intended in this Ayah: It is either the presence of ‘killing in this worldly life’ or punishment in the hell fire in the afterlife.

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Abridged and paraphrased. See Zaadul Maseer of Imaam Ibnul Jawzi (rahimahullaah)