Let $G$ be a Lie group with a closed subgroup $H$, and let $M$ be a smooth $H$-manifold. I am searching for a reference where it is proved that the tangent bundle of $G \times_H M$ is isomorphic to the bundle $TG \times_{TH} TM$ where $TH$ and $TG$ carry the tangent group structures.
I once stumbled upon a paper where this is proved in the beginning, but I cannot find it anymore. Does anybody know which paper I am talking about?