Reference for Bava Kamma 103:1

כיון שמסר לו מפתח קנה

as soon as he hands over the key to him, the conveyance is complete. What are the circumstances? If money was previously paid, why was the conveyance not completed by the money? If possession was taken, why was the conveyance not completed by possession? — We suppose that in fact possession was taken [of the house], and it was still requisite for the seller to say to the buyer, 'Go forth, take possession and become the owner', but as soon as he handed over the key to him, this was equivalent [in the eye of the law] to his saying to him, 'Go forth, take possession and complete the conveyance.'

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