面従後言 (menjūkōgen: pretending to obey someone to his face but badmouthing him behind his back)
face + to comply + behind + to say
Here’s a comparable expression using 背 instead of 後 (and occupying a different position in the compound):
面従腹背 (menjūfukuhai: pretending to obey but secretly betraying (someone))
face + to comply + abdomen + to disobey
The first expression contrasts behavior in front of one’s face with behavior done behind the back. The second expression contrasts conduct in front of the face with conduct somewhere in the realm of the abdomen!
