Mark 9:28

And when He had come into the house, His disciples asked Him privately, "Why could we not cast it out?"

Surrounding context

9:27 But Jesus took him by the hand and lifted him up, and he arose.

9:28 And when He had come into the house, His disciples asked Him privately, "Why could we not cast it out?"

9:29 So He said to them, "This kind can come out by nothing but prayer and fasting."