assure (one) of (something)
To try to convince one of something or quell one's fears. The partners are getting nervous—we need to assure them of the deal's favorable terms. Can you please assure the boss of my ability to handle this project?
assure someone of something
to guarantee something to someone; to promise someone that something will happen or that a particular state exists. I want to assure you of our good intentions. Frequently, she had to assure herself of her basic worth.