Etale cohomology of pushforward on affine scheme

Question

Let $X = Spec(\mathcal O_K)$ be the spectrum of the ring of integers of a number field $K$. Let $\mathfrak p$ be a non-zero prime ideal of $\mathcal O_K$ and let $i: Spec(\mathcal O_K/\mathfrak p) \to X$ be the inclusion map. Let $\mathbb Z$ be the constant sheaf on $Spec(\mathcal O_K/\mathfrak p)$. Apparently, we have $$H^1(X_{et}, i_* \mathbb Z) = H^1(Spec(\mathcal O_K/\mathfrak p)_{et}, \mathbb Z).$$ Could somebody please give me a hint or a reference for why this is true? Thank you.