  Let $F$ be a global field. Let $G$ and $H$ be two connected reductive group
defined over $F$ endowed with an $F$-morphism $f: H\rightarrow G$ such that the
induced morphism $H_{der}\rightarrow G_{der}$ on the derived groups is a
central isogeny. Our main results yield in particular the following theorem:
Given any irreducible cuspidal representation $\pi$ of $G(\mathbb A_F)$ its
restriction to $H(\mathbb A_F)$ contains a cuspidal representation $\sigma$ of
$H(\mathbb A_F)$. Conversely, assuming moreover that $f$ is an injection, any
irreducible cuspidal representation $\sigma$ of $H(\mathbb A_F)$ appears in the
restriction of some cuspidal representation $\pi$ of $G(\mathbb A_F)$. This
theorem has an obvious local analogue.
