It was narrated from Al-Aswad that Bilal said:
"The final words of the Adhan are: 'Allahu Akbar, Allahu Akbar; La ilaha illallah (Allah is the Greatest, Allah is the Greatest, there is none worthy of worship except Allah.)'" (AR)
English reference (AR): Vol. 1, Book 7, Hadith 650 (AR)
Arabic reference (AR): Book 7, Hadith 656 (AR)