Matthew 17:17

When they came to the multitude, a man came to him, kneeling down to him and saying,

“Lord, have mercy on my son, for he is epileptic and suffers grievously; for he often falls into the fire, and often into the water.

So I brought him to your disciples, and they could not cure him.”

Jesus rebuked the demon, and it went out of him, and the boy was cured from that hour.

Then the disciples came to Jesus privately, and said, “Why weren’t we able to cast it out?”

He said to them, “Because of your unbelief. For most certainly I tell you, if you have faith as a grain of mustard seed, you will tell this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.