Let $G$ be a connected semi simple Lie group with finite center. Fix a maximal compact subgroup $K$. An irreducible representation $(\pi,V)$ of $G$ is called a "class-one representation", if it contains non-zero $K$-fixed vectors. It is often treated as well-known, at least for splitrank one groups, that every irreducible unitary class-one representation is either trivial or infinitesimally equivalent to a representation induced from a minimal parabolic (and not only a sub representation of the latter as the sub representation theorem says).
Where can I find a proof of this "well-known"?