Change of availability

[PhoenixLight]

Legally, can an employer refuse a change of availability in my hours if it does not affect business?
If not could I have a link to something that says he can't

[buildersmate]

You've used slightly odd wording here - a 'change in availability in your hours' sounds like you have previously been working x am to Y pm and you've determined (for some perfectly valid reason, perhaps) that you can't do these hours anymore.
You asked for something that says he can't - this seems to imply you believe that you can change unilaterally decide to change your hours. Unless there is something in your contract to see you can do this, I'm afraid you can't.
You can ASK for a change in working hours, either temporarily to overcome a hiccup, or permanently, but the employer is under no obligation to agree to this.
And unfortunately it for the employer to decide whether it impacts his business or not.