I am studying from Chandler's book (Introduction to Modern Statistical Mechanics) the fluctuation-dissipation theorem. Before introducing it, the book states something without really demonstrating it. It says that the time correlation function of fluctuations goes to zero when the time difference between the two instants that appear in the formula goes to infinity, namely:
$$C(t)=\int dxf(x)\delta A(t';x)\delta A(t'+t;x) \rightarrow0 \quad as \quad t\rightarrow +\infty$$
where $\delta A(t)$ is the fluctuation at time $t$ of $A(t)$ from the time-independent average value $\langle A\rangle$, and $f(x)$ is the probability density function of phase space points. This comes from stating that for large $t$ we have $$\langle \delta A(t')\delta A(t'+t)\rangle\rightarrow\langle\delta A(t')\rangle\langle\delta A(t'+t)\rangle$$ I cannot justify this last statement. Where does it come from? Is it an axiom?