Luke is the only gospel that tells this famous parable.
A certain lawyer stood up, and tempted Jesus.
Lawyer: Master, what shall I do to inherit eternal life?
Jesus: What is written in the law? How do you read it?
Lawyer: You shall love the Lord your God with all your heart, and with all your soul, and with all your strength, and with all your mind; and your neighbor as yourself.
Jesus: You have answered right: do this, and you shall live.
Lawyer (wanting to justify himself): And who is my neighbor?
Jesus answered with a parable.
Jesus: A certain man went down from Jerusalem to Jericho, and fell among thieves, who stripped him of his clothing, wounded him, and departed, leaving him half dead. By chance a certain priest came down that way. When the priest saw the wounded man, he passed by on the other side. Likewise a Levite, when he was at the place, came and looked on him, and passed by on the other side. But a certain Samaritan journeyed and came where he was. When the Samaritan saw the wounded man, he had compassion for him. He went to him, and bound up his wounds, pouring in oil and wine, and set him on his own beast, and brought him to an inn, and took care of him. When he departed the next day, he took out two pence, and gave them to the host, and said to him, Take care of him; and whatever you spend more, I will repay you when I come again. Now which of these three, do you think was a neighbor to him that fell among the thieves?
Lawyer: He who showed mercy to him.
Jesus: Go, and do likewise.