When you hear the Adhan (call to prayer), repeat what the Muezzin says

The Prophet (may Allah's peace and blessings be upon him) is urging us here to respond to the Muezzin on hearing him by repeating what he says word for word. So, when he says: "Allāhu Akbar" (Allah is the Greatest), we should say: "Allāhu Akbar" after him; and when he pronounces the two testimonies of faith, we should pronounce them after him. The only exception is when he says: "Hayy ‘ala as-salāh, hayy ‘ala al-falāh"

(Come to prayer, come to success), we should say: "La hawla wala quwwata illa billāh" (There is no might or power except through Allah).