The Latin proverb Verba volant, scripta manent which literally it means "spoken words fly away, written words remain" is quite commonly used in Italian.
This phrase seems to come from a speech of Caius Titus of the Roman Senate, who suggests that spoken words might easily be forgotten, but written documents can always be conclusive in public matters.
A related, more common, meaning is that if two people want to establish a formal agreement about something, it is better to put it in writing, rather than just having an oral agreement.
The proverb doesn't necessarily suggest mistrust between the parties involved, but that clarity and points that have been discussed and could be forgotten or misunderstood after some time need to be fixed.
I recently used the proverb during a business conversation in English but I realized I was not understood so I had to explain it. I later checked and saw that it actually is not commonly used in English according to Ngram.
A usage example could be:
- "I suggest we write down our agreement as soon as possible, you know…verba volant."
Is there a common equivalent proverb or other idiomatic expressions in English to suggest the meaning stated above?