The reward is not specified. It could be related to something eschatological (see Matthew 25:34–36)1 or the blessing of a good conscience/knowledge that your actions have pleased your Father.2 In either case, the motive for action is not selfish (a heavenly reward) but simply a desire to please the Father because you love the Father.
4 afin que ton aumône se fasse en secret; et ton Père, qui voit dans le secret, te le rendra.