|| Matthew 17:14-21
MK9:25 As Jesus saw the crowd running to gather around, he rebuke the unclean demon, saying: “I command you, come out of him! Do not enter him any more!” MK9:26 And the demon screamed and convulsed the boy greatly and he became like a dead person. In fact, many said: “He is dead!” MK9:27 But Jesus took him by the hand and raised him up so that he stood up. MK9:28 Now later when they entered the house, his disciples asked him in private: “Why could we not exorcise [the demon]?” MK9:29 Jesus answered them: “This kind [of demon] will only come out through prayer.”
 Matthew 17:14-21: For details see notes in Nazarene Commentary 2000 on Matthew.
 Why could we not exorcise [the demon]: If we realize that three of the apostles were taken to an experience they could not relate, and now they return to the other eight who have failed in their attempt to exorcise the demon.
 Only come out through prayer: Since Jesus himself does not pray regarding the matter, he must mean the prayers of those disciples.