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).'" (ID)
English reference (ID): Vol. 1, Book 7, Hadith 653 (ID)
Arabic reference (ID): Book 7, Hadith 659 (ID)