Soloman is friends with some guy who had half his pay stolen by a demon and it keeps sucking his thumb. So Soloman is given a ring with a pentagram on it by Michael that can command demons. He commands thumb sucking demon to get Beelzebub and Solomon enslaves him. Now Solomon controls all demons and uses them to build a temple.

Got that from the wiki page for Testament of Solomon, there's more to it if you're interested.