thermodynamics - Fluctuations in Fluctuation Dissipation theorem

In the derivation of the Fluctuation-Dissipation theorem. We encounter an identity

$$ \langle\delta A(t) \delta B(0) \rangle = \langle A(t)B(0)\rangle-\langle A \rangle\langle B\rangle$$ where $$\delta A(t) = A(t) - \langle A\rangle $$ and $$\delta B(t) = B(t) - \langle B\rangle $$

$\langle\cdot\rangle$ denotes ensemble average.

How is this derived?