Lie derivative of an affine connection
= Lie derivative of an affine connection
The Lie derivative of a connection is the tensor
$$
(\mathcal L_X\nabla)_YZ
=\mathcal L_X(\nabla_YZ)-\nabla_{\mathcal L_XY}Z-\nabla_Y(\mathcal L_XZ).
$$
An isometry preserves the Levi-Civita connection, so $\mathcal L_K\nabla=0$ for a <Killing vector field>.