He who can does - he who cannot, teaches.
Context / explanation
from the Appendix to his play "Man and Superman: Maxims for Revolutionists", line 36
Evidences
1923: The Education Outlook, Volume 73, Page 29, Publisher S. Birch, Evidence
Similar quotesThose that know, do. Those that understand, teach.
Aristotle 
Experience teaches only the teachable.
Aldous Huxley 