´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.)'"
Explanation & Benefits
Hafiz Muhammad Ameen
650. Commentary: The purpose of recording the final words is so that no one, by analogy with the beginning, says «الله أكبر» four times, or by analogy with other words, says «لا إله إلا الله» twice, or adds "Ashhadu" at the beginning. Since these final words differ in style from the rest of the adhan, they have been specifically recorded.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 650