Isaiah 58:6-11:

Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?
Is it not to deal thy bread to the hungry, and that thou bring the poor that are cast down out to thy house? when thou seest the naked, that thou cover him; and that thou hide not thyself from thine own flesh?
Then shall thy light break forth as the morning, and thine health shall spring forth speedily: and thy righteousness shall go before thee; the glory of the LORD shall be thy rereward.
Then shalt thou call, and the LORD shall answer; thou shalt cry, and he shall say, Here I am. If thou take away from the midst of thee the yoke, the putting forth of the finger, and speaking vanity;
And if thou draw out thy soul to the hungry, and satisfy the afflicted soul; then shall thy light rise in obscurity, and thy darkness be as the noon day:
And the LORD shall guide thee continually, and satisfy thy soul in drought, and make fat thy bones: and thou shalt be like a watered garden, and like a spring of water, whose waters fail not.

Notes on Isaiah 58:6-11:

Here God says that showing His compassion on the earth will result in your health springing forth speedily.  God could not promise this if He were unwilling to do it for 100% of the people who keep the conditions in this passage.

This passage is usually misconstrued to indicate that fasting itself will break yokes, set the oppressed free, and so on.  However, that isn’t the point at all.  Isaiah is saying that instead of just starving yourself while being mean, God would prefer that you help others; this would be a more meaningful “fast” to Him that He could honor.  In this passage it is the Israelites, not God, who are supposed to take away the yokes (that THEY were putting on other people) and undo the heavy burdens, as you can see if you read the passage carefully in its context.

See also:

Notes on Psalm 41:1-3
Healing and Giving
Some of God’s Promises Have Conditions
Does Isaiah 58 Prove That If I Fast, My Health Will Spring Forth Speedily?
Condition: Broken Bones