contract/contract_sale/views/abstract_contract_line.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="account_abstract_analytic_contract_line_view_form"
            model="ir.ui.view">
        <field name="name">account.abstract.analytic.contract.line.form (in contract_sale)</field>
        <field name="model">account.abstract.analytic.contract.line</field>
        <field name="inherit_id"
               ref="contract.account_abstract_analytic_contract_line_view_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='discount']" position="attributes">
                <attribute name="groups">sale.group_discount_per_so_line
                </attribute>
            </xpath>
        </field>
    </record>

</odoo>