Is a prayer valid if the worshipper forgets an obligatory missed prayer (Fā'itah) during it?

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The prayer performed while forgetting a missed prayer is valid. Imam Ahmad has definitively stated this, ruling that if one remembers a missed prayer after completing the current one, the current prayer suffices, and the missed prayer must be made up. The justification rests upon the general saying of the Prophet that forgiveness is granted to the Ummah for error and forgetfulness. Furthermore, the forgotten prayer lacks a sign or indicator that it should have been performed, allowing forgetfulness to affect it, similar to fasting. This view is supported by the ruling that if one fears missing the time for the current obligatory prayer before being able to perform both, the requirement for sequence (Tartīb) is waived, and the current prayer takes precedence.