It was a private key, i found it in a forum.
Not in the darknet, in the clear Internet.
Best Regards!
You are very lucky. You shouldn't have posted about it here. This "40 BTC" in the title is definitely going to attract a lot of unwanted attention. And mind you, the wallet thieves are getting active once again, perhaps encouraged by the latest rally in the exchange rates. Move the coins to your personal wallet as soon as possible and guard against hacking.