EXCLUSIVE

The first man to make a legal weed purchase in the United States ... ever ... is doing the most un-stoner thing possible with his new stash -- he's locking it away in a safe.

32-year-old Sean Azzariti tells TMZ, he's storing his 3.5 grams of Bubba Kush, his truffle edible, and his $57 receipt from the Denver dispensary where he bought all the stuff because he plans to donate it to a museum.

"It's a piece of history, dude," Sean says.