The source and commentary distinguish the two:
• Fard: remaining seated for the required duration.
• Wajib: reciting the Tashahhud.
Therefore leaving the verbal Tashahhud is not described as omission of the Fard sitting itself.
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Sitting Is Fard; Reciting Tashahhud Is Wajib
The Final Sitting: Its Fard Duration and Its Classification
The source and commentary distinguish the two:
• Fard: remaining seated for the required duration.
• Wajib: reciting the Tashahhud.
Therefore leaving the verbal Tashahhud is not described as omission of the Fard sitting itself.