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Let $p : \mathcal{S} \rightarrow \mathcal{C}$ be a stack (or fibred category, I do not know if just being a fibred category is enough) over the site $\mathcal{C}$.

I have read somewhere (I forgot where but I think on "Stacks project") that the site $\mathcal{C}$ induces a Grothendieck topology on $\mathcal{S}$ (something to do with a definition of strongly Cartesian) but I forgot the link and also the proof.

Can somebody explain to me the proof of this result (or the link)?

Mathemagician
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Given an object $U$ of $\mathcal{S}$, I would declare a collection of arrows $\{f_\alpha:U_\alpha\rightarrow U\}$ in $\mathcal{S}$ to be a cover of $U$ in $\mathcal{S}$ if their image $\{p(f_\alpha):p(U_\alpha)\rightarrow p(U))\}$ is a cover for $p(U)$ in $\mathcal{C}$ in the sense of Grothendieck topology on $\mathcal{C}$.

This gives a Grothendieck topology on $\mathcal{S}$.