If he finds a pen on the desk and does not know who its owner is, what should he do with it?

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Check below answers in case you are looking for other related questions:
Rulings on lost property (al-luqtah)
A young child picked up a piece of gold that had been lost

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Related Answers:
He found a sheep in the desert – what should he do with it?
Ruling on picking up lost property in the Haram of
He found a watch in Makkah and he picked it up and left
Rulings on lost property (al-luqtah)
A young child picked up a piece of gold that had been lost

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