If he remembers a missed prayer during the current prayer or after finishing it

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When he was praying Zuhr, he remembered that he had not prayed Fajr
Should he pray the present prayer or the prayer he missed?

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Related Answers:
He does not remember the number of prayers and fasts that he owes. What should he do?
He did not know that ghusl from janaabah is obligatory; should he repeat the prayers?
He prayed for three years without doing ghusl from janaabah
When he was praying Zuhr, he remembered that he had not prayed Fajr
Should he pray the present prayer or the prayer he missed?

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