Bet you haven't seen this recently!
It happened after a solo Home Run by Domonic Brown of the Phillies.
In case you were wondering, spitballs aren't allowed.
Check out MLB Rule 8.02 (a) (2):
The pitcher shall not --
(2) expectorate on the ball, either hand or his glove
Thanks to Crashburn Alley for the gif. If you want Phillies coverage, that's the place to be!