We consider the problem of securing inter-flow network coding with multiple sources. We present a practical homomorphic signature scheme that makes possible to verify network coded packets composed of data originating from different sources. The multi-source signature scheme allows to circumvent the need of a secret key shared by all sources. Our solution is an extension of the pairing based homomorphic signature scheme by Boneh et al. We prove the security of the extended scheme by showing a reduction to the single-source case. We evaluated the performance of required computations and our results imply that the solution is applicable in practice.