When ṣilah applies to a pronoun hāʾ with ḍammah, a wāw-like lengthening sound follows the vowel. The additional sound need not be written as an ordinary full letter in the text.
This describes pronunciation, not an instruction to insert a new written character or to lengthen without limit.
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Ḍammah produces a wāw-like ṣilah
Rules of Hāʾ al-Ḍamīr: Vowels, Ṣilah and Exceptions
When ṣilah applies to a pronoun hāʾ with ḍammah, a wāw-like lengthening sound follows the vowel. The additional sound need not be written as an ordinary full letter in the text.
This describes pronunciation, not an instruction to insert a new written character or to lengthen without limit.