Check this free module too: https://apps.odoo.com/apps/modules/14.0/jt_amount_in_words/

Thank you for sharing this, it's exactly what I needed!

@Niyas Raphy
I have used this code for my invoice report, and it worked:
from odoo import fields, models

class InvoiceOrder(models.Model):
_inherit = 'account.move'

# @api.multi
def _compute_amount_in_word(self):
for rec in self:
rec.num_word = str(rec.currency_id.amount_to_text(rec.amount_total))

num_word = fields.Char(string="Amount In Words:", compute='_compute_amount_in_word')

Now I want to do a unit test for this function, and still don't know how to do it!