  Assembly bias describes the finding that the clustering of dark matter haloes
depends on halo formation time at fixed halo mass. In this paper, we analyse
the influence of assembly bias on galaxy clustering using both semi-analytical
models (SAMs) and observational data. At fixed stellar mass, SAMs predict that
the clustering of {\it central} galaxies depends on the specific star formation
rate (sSFR), with more passive galaxies having a higher clustering amplitude.
We find similar trends using SDSS group catalogues, and verify that these are
not affected by possible biases due to the group finding algorithm. Low mass
central galaxies reside in narrow bins of halo mass, so the observed trends of
higher clustering amplitude for galaxies with lower sSFR is not driven by
variations of the parent halo mass. We argue that the clustering dependence on
sSFR represent a direct detection of assembly bias. In addition, contrary to
what expected based on clustering of dark matter haloes, we find that low-mass
central galaxies in SAMs with larger host halo mass have a {\it lower}
clustering amplitude than their counter-parts residing in lower mass haloes.
This results from the fact that, at fixed stellar mass, assembly bias has a
stronger influence on clustering than the dependence on the parent halo mass.
