2 Corinthians 12:7-10 (New International Version)

7 or because of these surpassingly great revelations. Therefore, in order to keep me from becoming conceited, I was given a thorn in my flesh, a messenger of Satan, to torment me. 8 Three times I pleaded with the Lord to take it away from me. 9 But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly about my weaknesses, so that Christ’s power may rest on me. 10 That is why, for Christ’s sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficulties. For when I am weak, then I am strong.

From BibleGateway.com: http://www.biblegateway.com/passage/?search=2 corinthians 12:7-10&version=NIV