´Abu Al-Balj narrated from Muhammad bin Hatib Al-Jumahi who said that:` The Messenger of Allah said: "The distinction between the lawful and the unlawful is the Duff and the voice."
Hadith Referenceسنن ترمذي / كتاب النكاح عن رسول الله صلى الله عليه وسلم / 1088