As Jesus started on his way, a man ran up to him and fell on his knees before him. “Good teacher,” he asked, “what must I do to inherit eternal life?” “Why do you call me good?” Jesus answered. “No one is good—except God alone. You know the commandments: ‘You shall not murder, you shall not commit adultery, you shall not steal, you shall not give false testimony, you shall not defraud, honor your father and mother.’” “Teacher,” he declared, “all these I have kept since I was a boy.” Jesus looked at him and loved him. “One thing you lack,” he said. “Go, sell everything you have and give to the poor, and you will have treasure in heaven. Then come, follow me.” At this the man’s face fell. He went away sad, because he had great wealth. Jesus looked around and said to his disciples, “How hard it is for the rich to enter the kingdom of God!” The disciples were amazed at his words. But Jesus said again, “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 someone who is rich to enter the kingdom of God.” The disciples were even more amazed, and said to each other, “Who then can be saved?” Jesus looked at them and said, “With man this is impossible, but not with God; all things are possible with God.” Then Peter spoke up, “We have left everything to follow you!” “Truly I tell you,” Jesus replied, “no one who has left home or brothers or sisters or mother or father or children or fields for me and the gospel will fail to receive a hundred times as much in this present age: homes, brothers, sisters, mothers, children and fields—along with persecutions—and in the age to come eternal life. But many who are first will be last, and the last first.” – Mark 10:17-31
The primary reference used out of this text is in verse twenty-five where Jesus refers to the camel and the eye of a needle. Arguments on this reference range from a literal description of the size difference in the objects to a more contemporary reference using common day items of that time. Some even say that camel (greek word ‘kamilos’) was a misprint that should have been written ‘kamelos’ which means cable or rope. It could also be referring to a small door in the city wall of Jerusalem or some other city where the merchants’ camels would crawl through when the main gates were closed.
I have nothing against that specific line of verse but I don’t think it’s the climax of the point being made. What about Peter’s words “we have left everything to follow you!”? Jesus gave Peter a pat on the back by rephrasing his statement and underlining it in his “truly I tell you” response. Within their words is the key to the discovering what detachment is and why it is important. Detachment is leaving everything to follow Him. It is important because it’s right. Jesus was not speaking against the rich man’s richness but his attachment to the riches. Remember, Jesus said ‘How hard it is…’. In other words, you cannot attach to me if you are attached to something else and when you have lots of money it’s even more difficult to let go.
 At least, this seems to be the primary reference based on the teaching I have heard. I can’t remember very many times where this section of scripture was referenced that the teacher/preacher didn’t focus in on the ‘camel through the eye of a needle’ phrase.
 You can read more on detachment in Thomas Merton’s book Seeds of Contemplation. A lot of his thoughts in those chapters influenced my perspective on the topic.