Not quite. Alexander Fleming was growing cultures of staph bacteria, went away for some time, and when he came back, found one petri dish had been contaminated with fungus, and that the fungus inhibited the growth of the bacteria. It seems to be unclear where the contamination came from, but the fungus itself was already known to science at the time.

The cantaloupe is how the US production process in vats got a massive efficiency kick because the mould on the cantaloupe released significantly more penicillin than the prior one. But, this was after the initial discovery.

It's in "Florey: The Man Who Made Penicillin" by Lennard Bickel