In عَبَدْتُّمْ, a sākin dāl meets tāʾ. These consonants share an articulation zone but differ in qualities; at this specific place dāl merges into tāʾ as idghām mutajānisayn.
Do not confuse it with the ghunnah-bearing assimilation of tanwīn in «عَابِدٌ مَّا»; the triggers and articulation are different.
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ʿAbadttum: idghām of related consonants
Al-Kāfirūn and An-Naṣr: Applied Madd, Idghām, Ghunnah and Waqf
In عَبَدْتُّمْ, a sākin dāl meets tāʾ. These consonants share an articulation zone but differ in qualities; at this specific place dāl merges into tāʾ as idghām mutajānisayn.
Do not confuse it with the ghunnah-bearing assimilation of tanwīn in «عَابِدٌ مَّا»; the triggers and articulation are different.