See [Proposition 3.3, page 8, dJHS] and [Appendix B by Jack Hall, Theorem B.1, Smyth].

Proof. The absolute case follows from Artin's Axioms, Lemma 97.17.1 and Lemmas 98.15.4, 98.15.7, 98.15.6, 98.15.9, and 98.15.10. The case over $B$ follows from this, the description of $B\text{-}\mathcal{C}\! \mathit{urves}$ as a $2$-fibre product in Remark 98.15.5, and the fact that algebraic stacks have $2$-fibre products, see Algebraic Stacks, Lemma 93.14.3. $\square$

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).