God said to the Prophet David, "That servant is dearest to Me who does not seek Me from fear of punishment or hope of reward, but to pay the debt due to My Deity." And in the Psalms it is written, "Who is a greater transgressor than he who worships Me from fear of hell or hope of heaven? If I had created neither, should I not then have deserved to be worshipped?"
From "The Alchemy of Happiness" by Al Ghazzali, written in 1097 A.D.
From Chapter VIII: The Love of God. I heard this this morning on a podcast from Spiritual Classics.
You can find the whole text here.