RIHANNA Tour Buses Stopped at Border WEED FOUND

                 Rihanna's tour buses were stopped at the border between Michigan and Canada ... and law enforcement sources tell TMZ ... authorities found weed.

A total of 10 buses were stopped at the Ambassador Bridge, which separates Windsor, Canada and Detroit.  Inspectors smelled marijuana on one of the buses and initiated a secondary, bus-to-bus search.   

Inspectors then brought drug-sniffing dogs on board, and one of them made a beeline for a passenger and authorities found he was in possession of pot.  We're told he was cited with a civil penalty.

Rihanna was not on any of the buses.