word of mouth, by - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Orally, by one person telling another, as in They don't advertise; they get all their customers by word of mouth. [Mid-1500s]