19 Then the disciples having come to Jesus by himself, said, 'Why were we not able to cast him out?'
Matthew 17:19 Cross References - RYLT
Mark 4:10
10 And when he was alone, those about him, with the twelve, did ask him of the simile,
Mark 9:28
28 And he having come into the house, his disciples were questioning him by himself -- 'Why were we not able to cast it forth?'