set an example

Also, set a good or bad example. Behave in a way that should (or will) be imitated, as in Dad was always telling Bill to set a good example for his younger brother, or They were afraid of setting a bad example for the other nations. [Late 1700s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.