After reading some of the comments I thought of looking up if Graeber had actually said this quote. It looks like he did. In an interview in 2012: David Graeber: Beholden | Rebecca Solnit and David Graeber on anarchism as a problem-solving tool, the return of debtors’ prisons, and why communism is ingrained in capitalism. The full paragraph goes like that:

For example, the notion that anything that money touches, or that commerce touches, or that exchanges trade, is capitalism, and if you introduce any element of that into what might seem to be a non-capitalist situation, then suddenly that is part of capitalism. One of the things I discovered while writing this is that, to the contrary, markets and state power have always been deeply intertwined. And one of the facts that I discovered while researching, which I didn’t know at all before I started writing, is that free market ideology—does anyone know where it first comes from? It comes from medieval Islam, and specifically, Shari’a. Because Shari’a provided this commercial law that is independent from the state.