لَا يَحِلُّ لِمُسْلِمٍ أَنْ يَهْجُرَ أَخَاهُ فَوْقَ ثَلَاثٍ، فَمَنْ هَجَرَ فَوْقَ ثَلَاثٍ فَمَاتَ، دَخَلَ النَّارَ
A Muslim is not permitted to shun a fellow Muslim for more than three days. The narration then gives a grave warning concerning one who persists in such shunning and dies.
Source: أحمد وأبو داود
يَهْجُرَ أَخَاهُ means to shun a fellow Muslim or stop speaking to them; فَوْقَ ثَلَاثٍ means “for more than three.” The narration addresses estrangement driven by personal hostility. Its final clause gives a grave warning meant to correct conduct, not to establish the ultimate fate of a named individual.
In لَا يَحِلُّ لِمُسْلِمٍ أَنْ يَهْجُرَ, لَا يَحِلُّ indicates impermissibility, and أَنْ يَهْجُرَ describes the action. فَمَنْ introduces a conditional clause about the consequence.