The reward in view connects with the reward that was previously mentioned by Jesus earlier in his sermon (Matthew 5:12), a reward for following Christ/belonging to him. Most likely this refers to a future inheritance: life with God in glory. 1 In Matthew 6, Jesus will also make mention of a reward (Matthew 6:1, Matthew 6:4, Matthew 6:6, Matthew 6:18).
46 For if you love those who love you, what reward do you have? Do not even the tax collectors do the same?