The weed and the gun discovered during Soulja Boy's arrest this morning were inside a briefcase ... which allegedly belongs to the rapper ... law enforcement sources tell TMZ.

TMZ broke the story ... 21-year-old Soulja and 4 other men were arrested in Temple, GA early this morning ... after cops pulled over their Escalade because one of the lights on the vehicle was out.

Sources tell us ... when cops approached the car, they detected a "strong odor" of marijuana ... and searched the SUV.

We're told cops found a briefcase during the search, which they believe belongs to Soulja Boy ... and inside, officers claim they found a "substantial amount" of weed and a firearm.

Law enforcement sources tell us ... all 5 men were hauled to a nearby jail and booked for marijuana possession and a gun charge.