word to the wise, a
Here's good advice, as in A word to the wise: don't walk alone here because these streets are not safe at night. A shortening of A word to the wise is enough, as it was put by Roman writers, this phrase in English dates from the mid-1500s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.