John Gill Commentary Luke 9:40

"And I besought thy disciples to cast it out; and they could not." — Luke 9:40 (ASV)

And I besought your disciples
The nine disciples that were left behind, while Christ, and the other three, were gone up to the mountain:

to cast him out ;
the devil out of his child:

and they could not ;
(See Gill on Matthew 17:16).