What does Tom say to Legree that surprises him and sends a “thrill of amazement” through the other slaves?

Tom refuses to be "promoted" to a driver who whips slaves. Tom refuses to do this and is beaten for it. Still Tom says,

"I'm willin' to work night and day, and work while there's life and breath in me; but this thing I can't feel it right to do; and, mas'r, I never shall do it—never!"

This amazes the rest of the slaves.