Ṣah without tanwīn denotes a specified silence; ṣahin denotes an unspecified kind of silence. Both are imperative-like nouns of action, not ordinary declined nouns.
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Ṣah without and with Tanwīn
Tanwīn: Five Types, Their Function, and Omission
Ṣah without tanwīn denotes a specified silence; ṣahin denotes an unspecified kind of silence. Both are imperative-like nouns of action, not ordinary declined nouns.