Rihanna's tour buses were allegedly stopped and searched for drugs.

The 'Stay' singer - who recently kicked off her 'Diamonds' world tour - was not on any of the buses when they were reportedly stopped at the border between Michigan and Canada but a ''small amount'' of marijuana was discovered on an unidentified passenger.

A law enforcement source told gossip website TMZ.com that a total number of ten buses were stopped at the Ambassador Bridge, which separates Windsor, Canada and Detroit, and inspectors initiated a secondary bus-to-bus search after an officer smelt the drug on one of the buses.

A sniffer dog was brought on board and detected drugs on one passenger, who was then searched and a small amount of marijuana was discovered.

He was reportedly cited with a civil penalty.

The 24-year-old star resumed her tour last Thursday (14.03.13) after being forced to cancel two shows in Boston and Baltimore after falling ill with laryngitis immediately after the first show of the tour in New York state and tweeted she was ''embarrassed'' about it.

She wrote: ''BOSTON this is the hardest thing for me to deal with! I feel like we've been waiting on this day forever, and I'm hurt that I let you down (sic).

''I hate disappointing people that never ever let me down!! I'm so embarrassed about this! Thank you for your prayers and well wishes! (sic).''