It was narrated that Al-Aswad said: "The final words of the Adhan of Bilal were: “Allahu Akbar, Allahu Akbar, Allahu Akbar; La ilaha illallah (Allah is the Greatest, Allah is the Greatest, there is none worthy of worship except Allah)”"
Sahih IsnaadSahih IsnaadSahihBook 7 · Hadith 25