Power Shell

Hi,

I am trying to take a BackUp of a Site Collection from a remote computer, but I am getting a error that i am not sure of the script is

Invoke-Command -ComputerName remotecomputername -ScriptBlock {Backup-SPSite -Identity "http://remotecomputername:8080" -Path "C:\Users\username\Documents\SiteBackUp\BackUp8080"} -credential username

the error is get is :

The term 'Backup-SPSite' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the sp
elling of the name, or if a path was included, verify that the path is correct and try again.
    + CategoryInfo          : ObjectNotFound: (Backup-SPSite:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

Regards, Vikram