Looking on the internet deeply has found these results:

slap on his face is the most popular phrase on the web.

slap in his face 1208,000 results on the web	slap on his face 1315,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Have you decided which one of you is going to get to slap Barney right in his face? You deserve to slap someone in his face, as hard as you can. A slap in the face would be humiliating. Five minutes with tapley is a slap in the face. It's a slap in the face. That is a slap in the face. It's a slap in the face to our musicians. Well, I feel a little weird. I just met barney, and I don't have anything against the guy, but... how often do you get a chance to slap someone right in his face?	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

slap in his face

1208,000 results on the web

slap on his face

1315,000 results on the web

More popular!

Have you decided which one of you is going to get to slap Barney right in his face?
You deserve to slap someone in his face, as hard as you can.
A slap in the face would be humiliating.
Five minutes with tapley is a slap in the face.
It's a slap in the face.
That is a slap in the face.
It's a slap in the face to our musicians.
Well, I feel a little weird. I just met barney, and I don't have anything against the guy, but... how often do you get a chance to slap someone right in his face?

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.