Sunan an-Nasa'i, Volume 4, Book 26, Hadith 3371 (DarusSalam) ⓘ
It was narrated that Muhammad bin Hatib said: "The Messenger of Allah said: 'What differentiates between the lawful and the unlawful is the Duff, and the voice (singing) for the wedding.'"
أَخْبَرَنَا مُجَاهِدُ بْنُ مُوسَى، قَالَ حَدَّثَنَا هُشَيْمٌ، عَنْ أَبِي بَلْجٍ، عَنْ مُحَمَّدِ بْنِ حَاطِبٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " فَصْلُ مَا بَيْنَ الْحَلاَلِ وَالْحَرَامِ الدُّفُّ وَالصَّوْتُ فِي النِّكَاحِ " .
also referenced under: Hadith 3369 (Reference) & Book 26, Hadith 174 (In-Book)