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