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.

The real headline -- they only found a "small amount" of marijuana on one person in one bus. Who woulda thunk?