A promissory note is what is normally used but you should have had the friend sign before giving them the money. If they have the money they can refuse to sign the note.
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You should really have done this before lending him the money.
A simple contract should suffice. Make sure to list both party names, that you lent him $5000, and that he promises to pay you back. I recommend listing a date the $5000 has to be paid back by. Make sure both parties sign.
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