Forgetfully omitting Qunut in the third rakah of Witr invokes the Hanafi rules of sujud al-sahw because Qunut is treated as wajib. Omitting Qunut entirely differs from using another valid supplication.
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Forgetting Qunut in Witr
Witr Prayer: Three Rakahs, Qunut and Juristic Differences
Forgetfully omitting Qunut in the third rakah of Witr invokes the Hanafi rules of sujud al-sahw because Qunut is treated as wajib. Omitting Qunut entirely differs from using another valid supplication.