The Stacks project

Proof. Let $x, y$ be objects of $[U/R]$ over a scheme $T/S$. Let $x', y'$ be the images of $x, y$ in the category $[U'/R']_ T$. The functor $[f]$ is fully faithful if and only if the map of sheaves

is an isomorphism for every $T, x, y$. We may test this locally on $T$ (in the fppf topology). Hence, by Lemma 78.24.1 we may assume that $x, y$ come from $a, b \in U(T)$. In that case we see that $x', y'$ correspond to $f \circ a, f \circ b$. By Lemma 78.22.1 the displayed map of sheaves in this case becomes

This is an isomorphism if $R$ is the restriction, because in that case $R = (U \times _ B U) \times _{U' \times _ B U'} R'$, see Lemma 78.17.3 and its proof. Conversely, if the last displayed map is an isomorphism for all $T, a, b$, then it follows that $R = (U \times _ B U) \times _{U' \times _ B U'} R'$, i.e., $R$ is the restriction of $R'$. $\square$