Then Sir Arthur looked on the sword, and liked it passing well. Whether liketh you better, said Merlin, the sword or the scabbard? Me liketh better the sword, said Arthur. Ye are more unwise, said Merlin, for the scabbard is worth ten of the swords, for whiles ye have the scabbard upon you, ye shall never lose no blood, be ye never so sore wounded; therefore keep well the scabbard always with you. So they rode unto Carlion, and by the way they met with Sir Pellinore; but Merlin had done such a craft, that Pellinore saw not Arthur, and he passed by without any words. I marvel, said Arthur, that the knight would not speak. Sir, said Merlin, he saw you not, for an he had seen you, ye had not lightly departed.

Malory's Le Morte d'Arthur