Here is the story as told by one of his daughters:
When we finally arrived, the chauffeur escorted my younger sister, Laila, and me up to my father's suite. As usual, he was hiding behind the door waiting to scare us. We exchanged many hugs and kisses as we could possibly give in one day.
My father took a good look at us. Then he sat me down on his lap and said something that I will never forget. He looked me straight in the eyes and said, "Hana, everything that God made valuable in the world is covered and hard to get to. Where do you find diamonds? Deep down in the ground, covered and protected.
Where do you find pearls? Deep down at the bottom of the ocean, covered up and protected in a beautiful shell. Where do you find gold? Way down in the mine, covered over with layers and layers of rock. You've got to work hard to get to them."
He looked at me with serious eyes. "Your body is sacred. You're far more precious than diamonds and pearls, and you should be covered too."
Source: Taken from the book: More Than A Hero: Muhammad Ali's Life Lessons Through His Daughter's Eyes