John 16:7:

Nevertheless I tell you the truth; it is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.

See also:

Better
Your Authority over Sickness
Healing and Citizenship
How to Be Baptized with the Holy Spirit
What Does God Think About Taking Vitamins, Immune System Enhancers, Diet Aids, and So On?
Objection: Jesus Healed and Did Miracles to Prove His Deity
Objection: We Cannot Demand Healing from God Because God Is Sovereign
Objection: God’s Healing Covenant in the Law of Moses Was Only for the Jews
Objection: We Cannot Expect to Heal as Jesus Did Because We Are Not the Son of God
Objection: Faith Preachers Are Trinity-Denying Heretics Because They Teach That Jesus Died Spiritually