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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Check below answers in case you are looking for other related questions:
Rulings on changing the intention of a prayer after starting it
Ruling on the prayer of one who prays a fard prayer behind one who is praying a naafil prayer

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Related Answers:
Ruling on setting a time for the iqaamah for prayer so that it will be no later or earlier than a specific time
What is the reason why Qur’aan is recited out loud in some prayers and silently in others?
The right side of the row is better than the left
Rulings on changing the intention of a prayer after starting it
Ruling on the prayer of one who prays a fard prayer behind one who is praying a naafil prayer

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