I just had a thought:
When your mouth opens, your mind closes.
It’s a bit of a self-reminder to shut up occasionally and just listen. Like all pompous impressive-sounding aphorisms, it’s not 100% true … but it just may have a kernel of authenticity.
The interesting corollary that suggested itself to me is:
When your mind opens, your mouth closes.
Hrm … it’s just possible that no-one else has ever said the first version.