The Talmud relates a story about the great luminary, Rabbi Yehudah HaNasi. Arbata, the Parthian king sent to the Rabbi Yehudah a gift, a splendid pearl. Rabbi Yehudah responded with a gift of his own, a mezzuza. Outraged by the seeming mockery, the king angrily rebuked Rabbi Yehudah: "You have insulted me! I sent you a priceless gift, and you reciprocate with a trifle of no value?!"
"The gift you sent to me is so valuable that it will have to be guarded. The gift that I gave to you will guard you, even when you are asleep."
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