contract/contract_show_invoice/views/contract_view.xml

<?xml version="1.0"?>
<openerp>
    <data>

        <record id="act_analytic_invoices" model="ir.actions.act_window">
            <field name="name">Invoices</field>
            <field name="res_model">account.invoice</field>
            <field name="domain">[('invoice_line.account_analytic_id','=',active_id)]</field>
            <field name="view_id" ref="account.invoice_tree" />
            <field name="search_view_id" ref="account.view_account_invoice_filter" />
        </record>

        <record id="account_analytic_account_button_invoice" model="ir.ui.view">
            <field name="name">account.analytic.account.button.invoice
            </field>
            <field name="model">account.analytic.account</field>
            <field name="inherit_id" ref="analytic.view_account_analytic_account_form" />
            <field name="arch" type="xml">
                <xpath expr='//div[@name="buttons"]' position='inside'>
                    <button class="oe_stat_button" type="action" icon="fa-edit"
                        name="%(contract_show_invoice.act_analytic_invoices)d"
                        string="Invoices" help="Invoices related with this contract">
                    </button>
                </xpath>
            </field>
        </record>

    </data>
</openerp>