Old as the hills. Arlene Dahl wrote a beauty book with that suggestion in the fifties (only she recommended obtaining a fresh batch every day from your personal source), and she was probably a few thousand years late with that revelation.

Urine is also supposed to heal extremely dry skin. Some farmers and sailors pee on dry, cracked hands to help them heal. "Urea" is an ingredient in many medical skin creams for seriously dry skin.