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.
Related: NoSpecialPleadingForTheState.
See also: Taxation is necessarily theft. It can be nothing else., The Gun in the Room.