Unless you've added some of your own text to the S.A.L.L.I.E.3.0 Plugin, that phrase can be found at line 1973.
It appears to be in the DreamOutLoud portion but you can check that out for sure.
I think you could certainly change it to suit your likes but use care not to disturb the case or response portion.
If you are NOT using the S.A.L.L.I.E.3.0 Plugin then that phrase can be found in your halbrain.db file.
There are several occurrences of that phrase contained in there. I'm Afraid I can't do that, I'm afraid of this or that...and so on.