Yes, this is true. This was when Odysseus was disguising as a beggar during his return to the palace. Eurycleia was his old nurse and he knew that she would recognize the scar he has - and yes, she did while washing his feet. To stop Eurycleia from revealing his identity, he begged him not to tell anyone and even threatened her to die even though she was close to him. The old nurse immediately agreed to keep the secret.