The Bible teaches that saved people (those who have repented and trusted in the Savior for salvation) are promised rewards.
A side note: Salvation is not a reward given by God because salvation is a gift. However, after a sinner has been saved, his faithful service to God will be given rewards.
Question: Is it selfish and therefore sinful to aim for rewards in serving God?
Answer: God is the one who promised rewards to Christians who faithful serve Him. Therefore, aspiring for a reward in itself is not necessarily sinful.
The motivation of true Christian service is not primarily and solely to obtain a reward, although it is one of them. He serves God because he loves him. Love is his first and foremost motivation. Aiming for a reward is his secondary motivation.
He aims for a reward because through his rewards he aims to glorify God even more.
