The story of the rich young ruler that asked Jesus what he must do to have eternal life is found in Matthew 19:16-21 and Mark 10:17-27.
The rich young ruler came to Jesus asking what he needed to do to get eternal life. He assumed that it was by keeping the law perfectly, and he thought he had kept the law perfectly. But Jesus knew his heart and sought to teach him that he hasn’t been following the law perfectly and that by the young man’s standard he wasn’t saved. So Jesus told him all the things to do in the law which he thought he was doing, except for one thing that Jesus brought up that he wasn’t doing, showing the young man that he hadn’t been keeping the law perfectly. This was to get the young man to say to himself, “Uh oh. How can I be saved then if I’ve failed at keeping the law?” And then Jesus told him to do the right thing by using his rich abundance to supply the lack of others by selling his stuff and giving to the poor (not for salvation, but because he wouldn’t need those worthless possessions anymore and the poor always need help and that’s the right thing to do), and then to follow Him so that he could learn that salvation is through believing in Him and not by following the law.
Mark 10:17-27:
As He (Jesus) was setting out on a journey, a man (the rich young ruler) ran up to Him and knelt before Him, and asked Him, “Good Teacher, what shall I do so that I may inherit eternal life?” But Jesus said to him, “Why do you call Me good? No one is good except God alone (Yes, Jesus is good as He is God, but what He’s doing is setting up to show the man who wants to be justified by the law that only God is good and not humans per the law’s standard). You know the commandments: ‘Do not murder, Do not commit adultery, Do not steal, Do not give false testimony, Do not defraud, Honor your father and mother.’” And he said to Him, “Teacher, I have kept all these things from my youth.” Looking at him, Jesus showed love to him and said to him, “One thing you lack (here Jesus shows the young man that he failed to keep the law perfectly): go and sell all you possess and give to the poor (because it’s the right thing to do by using his rich abundance to supply the lack of others by selling his stuff and giving to the poor [not for salvation, but because he wouldn’t need those worthless possessions anymore and the poor always need help and that’s the right thing to do]), and you will have treasure in heaven; and come, follow Me (to learn that eternal life is gained by believing in Him rather than by following the law).” But he was deeply dismayed by these words, and he went away grieving; for he was one who owned much property. And Jesus, looking around, said to His disciples, “How hard it will be for those who are wealthy to enter the kingdom of God!” (Because most people trust in their works: “being a good person” rather than by faith in Christ alone) And the disciples were amazed at His words. But Jesus responded again and said to them, “Children, how hard it is to enter the kingdom of God! It is easier for a camel to go through the eye of a needle than for a rich person to enter the kingdom of God.” And they were even more astonished, and said to Him, “Then who can be saved?” Looking at them, Jesus said, “(Jesus coming full circle here:) With people it is impossible (following the law), but not with God; for all things are possible with God (His salvation through faith in the gospel).”