We address the problem of validating the integrity of data aggregated in applications that run on the Internet of Things (IoT). When data from multiple sources is sent to a single receiver, the receiver needs to efficiently verify the authenticity and integrity of data. For example, sensors from multiple IoT devices would send the measurements to a gateway for data aggregation and/or processing. The gateway needs to verify the authenticity and integrity of the readings from sensors. To address this need, we propose a lightweight homomorphic signature scheme that supports the execution of aggregation operations. Our approach collects digital signatures from multiple sources and provides an efficient aggregated signature verification mechanism for the end receiver. The aggregation and verification mechanism preserves the privacy of intermediate nodes. We discuss how our homomorphic signature scheme resists forgery and replay attacks.