King Artaban Sent a Pearl and Rabbi Judah Returned a Mezuzah

Artaban sent our teacher a priceless pearl. He said to him, "Send me something valuable in return." Rabbi Judah sent him a single mezuzah. Artaban said to him, "I sent you something that has no price, and you have sent me something worth one small coin?" He replied, "Your desirable things and my desirable things cannot be compared to her" (compare Proverbs 8:11). "And not only that, but you sent me something that I must guard, while I sent you something that, when you lie down, guards you, as it is said, 'When you walk it shall lead you'—in this world; 'when you lie down it shall watch over you'—at the hour of death; 'and when you awake it shall talk with you' (Proverbs 6:22)—in the world to come."