He was praying a naafil prayer, then some other people prayed behind him, so he changed his intention to make it an obligatory prayer

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Ruling on the prayer of one who prays a fard prayer behind one who is praying a naafil prayer
Rulings on changing the intention of a prayer after starting it

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Related Answers:
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Ruling on the prayer of one who prays a fard prayer behind one who is praying a naafil prayer
Rulings on changing the intention of a prayer after starting it

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