In the Yuddha Kanda of the Valmiki Ramayana, Vibhishana advises his brother Ravana, in the presence of the ministers, to return Sita to Rama. Ravana is enraged and dismisses him. The text says that Ravana “sent away Vibhishana, who had spoken honestly.”
Vibhishana goes over to Rama. He explains: “Reviled by him and humiliated as a slave, I, leaving my sons and my wife, have come to take refuge with Rama.”