How to Validate users by SqlDataSource Control?

How to Validate users by SqlDataSource Control?

I make a Login page to validate user. I have a sql table like following:
table: tblUsers
userID (int is identity)
username (nvarchr (50))
password (nvarchr (50))
Login.ASPX
Username<asp:TextBox ID="txtUsername" runat="server"></asp:TextBox><br />
Password <asp:TextBox ID="txtPassword"
runat="server"></asp:TextBox><br />
<asp:Button ID="cmdLogin" runat="server" Text="Login"
onclick="cmdLogin_Click" /><br />
<asp:Label ID="lblError" runat="server"
EnableViewState="False"></asp:Label><br />
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:ConStr %>"
SelectCommand="SELECT * FROM [tblUser] WHERE (([username] =
@username) AND ([password] = @password))">
<SelectParameters>
<asp:ControlParameter ControlID="txtPassword" Name="username"
PropertyName="Text" Type="String" />
<asp:ControlParameter ControlID="txtPassword" Name="password"
PropertyName="Text" Type="String" />
</SelectParameters>
</asp:SqlDataSource>
Validate users is in SelectCommand of SqlDataSource.
SELECT * FROM [tblUser] WHERE (([username] = @username) AND ([password] =
@password))
how to validate user by SqlDataSource? I want code for Login button.