The source example وَمَا هُمْ بِمُؤْمِنِينَ contains a sākin mīm at the end of هُمْ, immediately followed by ب at the start of بِمُؤْمِنِينَ. The book classifies the connected form under ikhfāʾ.
When joining the words, do not read an invented vowel as “humu,” and do not discard the mīm. Isolate the mīm first, then join the words while preserving the required nasal quality.
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Source example before bāʾ
The source example وَمَا هُمْ بِمُؤْمِنِينَ contains a sākin mīm at the end of هُمْ, immediately followed by ب at the start of بِمُؤْمِنِينَ. The book classifies the connected form under ikhfāʾ.
When joining the words, do not read an invented vowel as “humu,” and do not discard the mīm. Isolate the mīm first, then join the words while preserving the required nasal quality.