The reward is an inheritance.
Colossians 3:24 since you know that you will receive an inheritance from the Lord as a reward. It is the Lord Christ you are serving.
The reward is the promise of the inheritance of eternal life.
Hebrews 9:15
For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant.
In case you still are not sure the inheritance is eternal life, which is our reward, read and study these scriptures, there should be no more doubt after reading them.
Matthew 19:29 And everyone who has left houses or brothers or sisters or father or mother or wife or children or fields for my sake will receive a hundred times as much and will inherit eternal life.
Luke 10:25[ The Parable of the Good Samaritan ] On one occasion an expert in the law stood up to test Jesus. “Teacher,” he asked, “what must I do to inherit eternal life?”
The REWARD is the PROMISE, of an INHERITANCE, and the INHERITANCE is ETERNAL LIFE.