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 " .
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).'" Translation Not Available
Book Ref: Sunan An Nisai Book 26 Hadith 3370
Web Ref: Sunan An Nisai Vol 4 Book 26 Hadith 3372