Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
word of mouth n
by or through word of mouth из уст в уста́
I found out about it by word of mouth я об э́том услы́шал от кого́-то
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: