Romans 8:28:

And we know that all things work together for good to them that love God, to them who are the called according to his purpose.

See also:

Objection: Job Was a Godly Man, Yet God Let Him Get Sick
Objection: Sickness Is a Blessing in Disguise
Objection: We Cannot Demand Healing from God Because God Is Sovereign
Objection: God Works All Things for Good (Romans 8:28); This Includes Sickness
Objection: God Uses So-and-So Powerfully Because She Was Paralyzed When She Was Young