2 Corinthians 12:7:

And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.

See also:

The Baseball Diamond of Healing
Objection: God Wouldn’t Heal Paul’s Thorn in the Flesh
Objection: Faith Does Not Always Overcome – God Can Give Both OVERCOMING and ENDURING Faith
Objection: It Is Unbalanced to Teach That Life’s Problems Are Due to a Lack of Faith
Objection: Since God Allows Persecution, Which Can CAUSE Sickness, He Allows Sickness