The data content in ChEMBL is licensed under a highly permissive Creative Commons license - specifically the "CC Attribution-ShareAlike 3.0 Unported license"
The required attribution should contain the url of the ChEMBL resource, and also the release version, e.g.:
ChEMBL data is from http://www.ebi.ac.uk/chembl - the version of ChEMBL is ChEMBL_07.
This should be visible on the entry portal for a web resource in which ChEMBL is integrated, or contained with the documentation for any further distribution.