Matthew 17:20-21

He said to them, Because of the littleness of your faith [that is, your lack of firmly relying trust]. For truly I say to you, if you have faith [that is living] like a grain of mustard seed, you can say to this mountain, Move from here to yonder place, and it will move; and nothing will be impossible to you. But this kind does not go out except by prayer and fasting.