By using javascript you can do it
function NumOnly(ctrl) {
chr = document.getElementById(ctrl).value;
var reg1 = /.*[a-zA-Z].*/;
if (document.getElementById('<%=txtMobile.ClientID %>').value == "")
return;
if (reg1.test(chr)) {
document.getElementById(ctrl).value = "";
alert("Enter numbers only");
document.getElementById(ctrl).focus();
}
}
<asp:TextBox ID="txtMobile" runat="server" Width="159px" TabIndex="17" onkeypress='NumOnly(this.id)' MaxLength="10"></asp:TextBox>