take word for

take (one's) word for it

To accept what one says without further verifying or investigating. You're right to be wary, but, in this case, I think we can take John's word for it. He has no reason to lie to us.
See also: take, word

take someone's word for something

 and take someone's word on something
to believe what someone says about something without seeking further information or proof. It's true! Take my word for it. I can't prove it. You will have to take my word on it.
See also: take, word