A closed manifold of negative Ricci curvature has no conformal vector fields

Question

I am trying to find a reference for the following claim, which is probably well-known:

Let $M$ be a closed manifold of dimension $\ge 3$, with negative Ricci curvature. Then every conformal vector field of $M$ is zero.

Any help would be appreciated.

The isometric case follows from Bochner's formula.

Answer 1

This is Theorem 1 of Yano's On Harmonic and Killing Vector Fields.