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
Imagine a criminal who didn’t have to rob you. Imagine he had so much might and threat, that all he had to do was maintain the threat of violence over you to make you hand your money over on a regular basis. Some people wouldn’t comply, and a lot might slip through the cracks, but every now and then he’d bring the fist down and make an example of someone to bring the slaves back in line again. The more public the protester and the more viscous the punishment, the better. Now that’s efficient thievery! Who says the government isn’t efficient? Nonsense. It may seem inefficient to the victim but it’s very efficient for those on the receiving end! —Dale Everett, Anarchy in your Head
Taxation is Theft with refutations of common objections.
Sophisticated Slavery, winner of an essay contest to best explain that taxation is theft.