Wireless ad hoc networks enable the wireless devices to directly communicate with each other. An emerging application in such systems is video-on-demand service that can greatly reduce the content server's workload by utilizing the nodes' resources. Such an application relies on the cooperation of all participating nodes. However, the nodes are selfish in nature. Therefore, a key design issue is to appropriately incentivize the cooperation among each other. In this paper, we design a simple yet effective reward based incentive mechanism so as to stimulate the nodes to upload and/or forward data to one another. By using a Stackelberg game model, we analyze the interactions between the content provider's rewarding strategy and the nodes' contributing behaviors. We derive a unique Stackelberg equilibrium and show its efficiency. By using a repeated game, we study the long term intelligent interaction between nodes and the content provider. We also design a cheating prevention mechanism and analyze its effectiveness under the repeated game setting.