Dharana is the sixth limb of Yoga and is generally known as concentration.  Before looking for any scriptual backing, it would be best to explain what Dharana is:

Yoga Sutra 3.1 One-pointedness is steadfastness of the mind.

Dharana is the ability to focus on one object to the exclusion of everything else.  Have you ever been so focused on a book, television show or movie that the rest of the world seems to vanish?  Dharana is that same kind of focus but it is focused on a single object and it is voluntary instead of unintentional.  The object itself is not important, only the ability to focus, to concentrate, is important.  Some use a candle flame, others a favorite image or even a word or sound will do.  Many confuse Dharana with Dhyana or meditation, while they are related they are not the same.  Dharana is simply, hah!, taming our thoughts so we can focus on a single thought where Dhyana takes this one step further as will be shown later.

7 For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.
Proverbs 23:7

Our thoughts define who we are; if we have no control of our thoughts then we have no control of ourselves.

28 He that hath no rule over his own spirit is like a city that is broken down, and without walls.
Proverbs 25:28

This reinforces the previous verse by pointing out that lack of control over our thoughts or spirit leaves us vulnerable like a city that is broken down and withouth walls.

5 Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;
2 Corinthians 10:5

Paul is speaking of a spiritual warfare where the enemy is to be found in our own minds and that victory is to be found by bringing every thought to focus on Christ.

3 Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee.
Isaiah 26:3

God will keep us in perfect peace if we keep our thoughts focused on Him because we will trust Him.

37 Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.
Matthew 22:37

Finally, we are commanded to serve God with all our minds.  How can we hope to accomplish this if we can’t keep our focus?

Namasté