We learned the story in Sunday School.

Jesus and his inner circle — Peter, the rock, and James and John, the sons of thunder — came down from the Mount of Transfiguration and found the disciples facing their own inability to cast an unclean spirit out of a young boy. Annoyed with the disciples, Jesus says to the father: “If you believe, all things are possible to him who believes.” (Mark 9:23) The father replies: “Lord, I believe; help my unbelief.” (Mark 9:24) Jesus casts out the unclean spirit. Later, when the disciples ask why they were unable to do it, Jesus says that some only come out through prayer and fasting.