Sunan an-Nasa'i, Hadith 3370 ⓘ
It was narrated that Abu Balj said: "I heard Muhammad bin Hatib say: 'What differentiates between the lawful and the unlawful is the voice (singing).'"
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، عَنْ شُعْبَةَ، عَنْ أَبِي بَلْجٍ، قَالَ سَمِعْتُ مُحَمَّدَ بْنَ حَاطِبٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّ فَصْلَ مَا بَيْنَ الْحَلاَلِ وَالْحَرَامِ الصَّوْتُ " .
also referenced under: Book 26, Hadith 175 (In-Book) & Volume 4, Book 26, Hadith 3372 (DarusSalam)