Do you imply ill intent upon the program? Because I read this as a violation of tone.
I interact with the program because I want/need to. It's in my interest to follow through the *instructions* to interact with it correctly and efficiently. I can follow instructions and view it as a helpful hint.
How about entering, say, a printer configuration. These fields accept only SSIDs, IP addresses etc?
No. Never tell me what I "need to" do.
There are any number of ways to get this message across without telling me what I need to do.
One example off the top of my head, probably not the best, but certainly an improvement because it doesn't blame me:
> This field only accepts numbers
For bonus points, put that message next to the field modelessly at the moment I type a non-numeric character.