Kerodon

Proposition 4.3.6.1 (Joyal [MR1935979]). Let $K$ be a simplicial set, let $\operatorname{\mathcal{C}}$ be an $\infty $-category, and let $f: K \rightarrow \operatorname{\mathcal{C}}$ be a diagram. Then the projection map $\operatorname{\mathcal{C}}_{f/} \rightarrow \operatorname{\mathcal{C}}$ is a left fibration of simplicial sets, and the projection map $\operatorname{\mathcal{C}}_{/f} \rightarrow \operatorname{\mathcal{C}}$ is a right fibration of simplicial sets. In particular, the simplicial sets $\operatorname{\mathcal{C}}_{f/}$ and $\operatorname{\mathcal{C}}_{/f}$ are $\infty $-categories (see Remark 4.2.1.4).