How To Solve Javascript Error - Using ToolScriptManager For UpdatePanel

I am trying to register below mentioned javascript for using with GridView to add freeze functioality on GridView.
When compiling an error is thrown Microsoft JScript runtime error: 'Sys' is undefined

How it can be fixed.

<script language="javascript" type="text/javascript">
// This Script is used to maintain Grid Scroll on Partial Postback
var scrollTop;
//Register Begin Request and End Request
Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
//Get The Div Scroll Position
function BeginRequestHandler(sender, args)
{
var m = document.getElementById('divGrid');
scrollTop=m.scrollTop;
}
//Set The Div Scroll Position
function EndRequestHandler(sender, args)
{
var m = document.getElementById('divGrid');
m.scrollTop = scrollTop;
}
</script>