With connection in فَوَيْلٌ لِلْمُصَلِّينَ, tanwīn is followed immediately by lām, so the nūn sound assimilates into lām without ghunnah. Holding a two-count nasalization here would be incorrect.
The article lām inside li-l-muṣallīn is a different matter, since mīm is a moon letter.
Contrast stopping after fawaylun with linking the two words: the cross-word idghām only exists in connected recitation.
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Fawaylun li-l-muṣallīn: assimilation without ghunnah
Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules
With connection in فَوَيْلٌ لِلْمُصَلِّينَ, tanwīn is followed immediately by lām, so the nūn sound assimilates into lām without ghunnah. Holding a two-count nasalization here would be incorrect.
The article lām inside li-l-muṣallīn is a different matter, since mīm is a moon letter.
Contrast stopping after fawaylun with linking the two words: the cross-word idghām only exists in connected recitation.