Let $(M, \tilde{g})$ be a Riemannian manifold and let $g:=e^{2u}\tilde{g}$ be a conformal change. I'm trying to find a resource (ideally a book, or a paper where it's mentioned or derived) for the formula for the sectional curvature under this conformal change.
For any $\tilde{g}$-orthonormal pair $e_1,e_2$ spanning a two-plane $\Pi \subset T_y \widetilde{M}$, $$K_g(\Pi) = e^{-2u} \Big( K_{\tilde{g}}(\Pi) - \mathrm{Hess}_{\tilde{g}}u(e_1,e_1) - \mathrm{Hess}_{\tilde{g}}u(e_2,e_2) + (du(e_1))^2 + (du(e_2))^2 - \|\nabla_{\tilde{g}}u\|^2 \Big).$$
I'd appreciate if you could please mention a book/paper that derives this formula so I can put in the bibliographic references.