If taxation without consent is not robbery, then any band of robbers have only to declare themselves a government, and all their robberies are legalized. —Lysander Spooner
"Taxation is theft" is perhaps more common, but specifically it's more like extortion: the state sends you a bill (or automatically takes it), and threatens harm if you don't pay. When the mob does this—even if they say they're providing "protection"—we call it extortion.
You'll have difficulty getting people to admit this (but it's easy enough to understand), especially to those who derive income from the state:
It is difficult to get a man to understand something, when his salary depends upon his not understanding it! —Upton Sinclair
Related: NoSpecialPleadingForTheState.
See also: Taxation is necessarily theft. It can be nothing else., The Gun in the Room.