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. 3
1 “Beware of practicing your righteousness before other people in order to be seen by them, for then you will have no reward from your Father who is in heaven.