By word of mouth
Definition 1 of 1
Pro Tip 1/3Informal Context
Used in casual conversations, not in formal reports or official announcements. 
I found this great cafe by word of mouth from friends.
Pro Tip 2/3Personal Trust
Implies a level of trust or endorsement when shared between individuals. 
He’s a reliable mechanic, I know him only by word of mouth from satisfied customers.
Pro Tip 3/3No Official Record
There's usually no documentary evidence when information is communicated by word of mouth. 
Many family recipes are passed down by word of mouth, without written instructions.