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.
take someone's word for somethingand 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.