Confirm Messge in GridView Button On RowDataBound using VB.Net in ASP.Net

Hi,

When i press cancel button in confirm box, then also my codes executes on Gridview RowCommand, how to avoid if i press cancel button.

  <asp:GridView ID="gridData" runat="server" AutoGenerateColumns="False" DataKeyNames="ID" class="table table-striped table-bordered"
                                OnRowDataBound="OnRowDataBound" Visible="False" AllowPaging="true" PageSize="5" OnPageIndexChanging="OnPaging" PagerStyle-CssClass="gridfooter">
                                <Columns>

                                    <asp:ButtonField ButtonType="Image" CommandName="XYZ" HeaderText="ABC" ImageUrl="~/images/123.png">
                                        <HeaderStyle ForeColor="Red" />
                                    </asp:ButtonField>
                                </Columns>

                                <PagerSettings PageButtonCount="15" />

                                <PagerStyle CssClass="gridfooter"></PagerStyle>

                            </asp:GridView>

If e.Row.RowType = DataControlRowType.DataRow Then
            e.Row.Cells(0).Attributes("onclick") = "return confirm('Are you sure want to send SMS?');"
        End If

Protected Sub gridData_RowCommand(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles gridData.RowCommand
       If e.CommandName = "XYZ" Then
            ltrCustID.Text = gridData.DataKeys(e.CommandArgument).Value
            SendSlip(ltrCustID.Text)
        End If
    End Sub