From 《三国演义》 Chapter 105, where a minister warns the emperor: 今日若听此一面之词,杨仪等必投魏矣 — 'if Your Majesty listens to only this one side's words, Yang Yi and the others will certainly defect to Wei.' The phrase has stayed fixed in legal and journalistic discourse.
usage
Almost always paired with verbs like 听 (listen to), 偏信 (believe one-sidedly), 轻信 (rashly trust). Stock phrase for journalism, law, and interpersonal disputes.