by hearsay

(know sth) by hearsay — знать что-л. от других (людей), понаслышке

Example: In case the speaker had not seen the sick person himself, he would have to express whether he knows by hearsay or by evidence that the person is sick, or whether he has dreamed it.

hearsay (unverified information heard or received / acquired from another; gossip; rumour)

[word of mouth]
[сарафанное радио ]
[on the grapevine]
[из уст в уста]