slap on the back
A gesture of congratulation, as in The coach gave him a slap on the back for coming in first. [Early 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for slap on the back
The garrulous or impertinent talker is almost as objectionable as the hail-fellow-well-met, slap-on-the-back fellow.The Book of Business Etiquette