First, what version of OS X are you using?
Secondly, what exact Mac model do you have?
Now, do you happen to have the file "Install OS X "whatever"" someplace on your Mac? The "whatever" would be the name of the OS you are using. For example, I have the file "Install OS X El Capitan" file in two locations on each of my Macs (along with two external locations each, via backups). If you have that file someplace, then you can use it, instead of re-downloading it.