ANSWER: The "covet" commandment in question is found in Exodus 20. It was given when God gave his holy law, through Moses, to the children of Israel.
"You shall (the KJV Bible has 'Thou Shalt') not covet your neighbor's house; you shall not covet your neighbor's wife . . ." (Exodus 20:17)
Can you imagine what would result if all the people on our planet would keep even one of God's laws, such as the one the forbids we covet someone's else's stuff? This would remove a whole host of worry and tragedy in our lives. The Commandments were given for our GOOD and are not meant to destroy us. The next time you read the newspaper or access news on the Web, note how people are hurt, suffer loss and so on by someone (either themselves or others) not obeying God's holy laws.
Jesus had a very interesting exchange regarding the commandments with someone who coveted wealth. Christ actually offered the man a chance to follow him, like his disciples did, but was turned down! The wealthy man was not willing to give up his possessions to follow the Savior of mankind (Matthew 19:16 - 24). He loved his wealth more than God. This is a clear violation of the Tenth commandment that forbids us to covet.
We are also clearly told by the Apostle Paul that when we covet we can also cause a violation of the First and Second commandments against idolatry. He stated, "Therefore, put to death your members which are on earth - sexual immorality, uncleanness, inordinate affection, evil desires, and covetousness, which is idolatry" (Colossians 3:5).
Allowing material things to become more important than our Creator, and to covet them in our lives, makes us guilty of idolatry (Luke 12:15). No real Christian would want that because, if it is not repented of, it would cut them off from God. Perhaps this is why there are so many warnings in the Bible against pursuing wealth. It is very difficult to be pursuing wealth and have time for the things of God.
It is also very difficult to have attained wealth and not set your heart on it. A few people in the Bible, however, have been what we would call wealthy yet dedicated to the Lord like Abraham and Job (Genesis 13:2, Job 1:1, 3).
It is unwise to covet riches and think we can stay focused on God's purpose for our life. We are taught in the Sermon on the Mount that our lives are to be directed towards God's kingdom (Matthew 6). We cannot serve our heavenly Father and mammon (money, riches, material possessions).
No servant is able to serve two masters; for either he will hate the one, and he will love the other; or he will hold to the one and will despise the other. You cannot serve God and mammon (Luke 16:13 - 14, HBFV, see also Mark 4:19)
Wise King Solomon, at the end of the book of Ecclesiates, makes a simple but profound statement. He sums up all the wisdom he gave in his book by stating the wisest thing anyone can do, the key to living life to the fullest, is obey God (which includes his command not to covet, see Ecclesiastes 12:13 - 14). Obedience is the key to loving God and loving our fellow humans (Matthew 22:35 - 40, see also 1John 4:21).