The news spread by word of mouth.

这消息是口头传开的。

《牛津词典》

The story has been passed down by word of mouth.

这个故事是口头流传下来的。

《柯林斯英汉双解大词典》

At the very beginning, information spread by word of mouth.

最开始的时候，信息是通过口口相传的。

youdao

There are other approaches as well such as those described by the Word of Mouth Marketing Association.

FORBES: Can You Measure Your Digital Eminence?

This means trying to spread the message by word of mouth still considered the most-powerful form of advertising.

ECONOMIST: The future of advertising

Consequently, for the first time since the days of the corner store (that was sustained or broken by a local reputation and word of mouth), virtually every brand must do what it says and act in the best interest of its customer set.

FORBES: Are Brands Entering An Era Of 'Truth In Advertising?'