What did Job refuse to do despite his suffering?

Answer: Curse God

Urged to curse God and die, Job replied, "Should we accept from God only good and not adversity?" He did not sin.

“Then Job’s wife said to him, “Do you still retain your integrity? Curse God and die!” “You speak as a foolish woman speaks,” he told her. “Should we accept from God only good and not adversity?” In all this, Job did not sin in what he said.”— Job 2:9-10 (BSB)