When he was praying Zuhr, he remembered that he had not prayed Fajr

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Check below answers in case you are looking for other related questions:
If he remembers a missed prayer during the current prayer or after finishing it
How should missed prayers be made up?

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Related Answers:
Should he pray the present prayer or the prayer he missed?
She is confused about purity following menses; what should she do?
Is it prescribed to make up the regular Sunnah prayers that should be offered before and after Zuhr by offering them after ‘Asr because of an excuse?
If he remembers a missed prayer during the current prayer or after finishing it
How should missed prayers be made up?

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