Sunan an Nasai
The Book of the Adhan (The Call to Prayer)
Hadith #27
7.27
أَخْبَرَنَا سُوَيْدٌ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ، عَنْ يُونُسَ بْنِ أَبِي إِسْحَاقَ، عَنْ مُحَارِبِ بْنِ دِثَارٍ، قَالَ حَدَّثَنِي الأَسْوَدُ بْنُ يَزِيدَ، عَنْ أَبِي مَحْذُورَةَ، أَنَّ آخِرَ الأَذَانِ، لاَ إِلَهَ إِلاَّ اللَّهُ .
It was narrated that Muharib bin Dithar said:"Al-Aswad bin Yazid narrated to me from Abu Mahdhurah that the final words of the Adhan are: 'La ilaha illahha (there is none worthy of worship except Allah)