St Matthew 17:14-20

And when he was come to the multitude, there came to him a man falling down on his knees before him, saying: Lord, have pity on my son, for he is a lunatic, and suffereth much: for he falleth often into the fire, and often into the water. And I brought him to thy disciples, and they could not cure him. Then Jesus answered and said: O unbelieving and perverse generation, how long shall I be with you? How long shall I suffer you? bring him hither to me. And Jesus rebuked him, and the devil went out of him, and the child was cured from that hour. Then came the disciples to Jesus secretly, and said: Why could not we cast him out? Jesus said to them: Because of your unbelief. For, amen I say to you, if you have faith as a grain of mustard seed, you shall say to this mountain, Remove from hence hither, and it shall remove; and nothing shall be impossible to you. But this kind is not cast out but by prayer and fasting.