When was the first time God promised eternal life in the Bible?

One of the central tenets of the Bible is that you can enter heaven if you follow the way of the Lord, living harmoniously with him. At some point of time (If not from the very beginning of the Bible), the promise/concept of eternal life was said to be one of the reward for someone who pleased the Lord. When exactly did this happen? When was the first time Mankind was promised eternal life as a reward?