In many location-based services, the user location is determined on the mobile device and then shared with the service. For this type of interaction, a major problem is how to prevent service abuse by malicious users who lie about their location. This paper proposes LINK (Location verification through Immediate Neighbors Knowledge), a location authentication protocol in which users help verify each other's location claims. This protocol is independent of the wireless network carrier, and thus works for any third-party service. For each user's location claim, a centralized Location Certification Authority (LCA) receives a number of verification messages from neighbors contacted by the claimer using short-range wireless networking such as Bluetooth. The LCA decides whether the claim is authentic or not based on spatio-temporal correlation between the users, trust scores associated with each user, and historical trends of the trust scores. LINK thwarts attacks from individual malicious claimers or malicious verifiers. Over time, it also detects attacks involving groups of colluding users.