There are many unsolved problems in ad hoc networks; security being one of the major concerns. The wormhole attack is among the most threatening and dangerous attacks on these types of networks. During the attack a malicious node captures packets from one location in the network, and tunnels them to another distant malicious node, which replays them locally. In this paper, we propose a scheme for the wormhole attack prevention. The scheme relies on the idea that usually the wormhole nodes participate in the routing in a repeated way as they attract most of the traffic. Therefore, each node will be assigned a cost depending in its participation in routing. Besides preventing the network from the wormhole attack, the scheme provides a load balance among nodes to avoid exhausting nodes that are always cooperative in routing.