Mukhtasar Muslim

Sayyiduna Umar bin Khattab (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: When the muezzin says "Allahu Akbar, Allahu Akbar," the listener should also repeat these words, and when he says "Ashhadu an la ilaha illallah" and "Ashhadu anna Muhammadan Rasulullah," the listener should also repeat these words. And when the muezzin says "Hayya 'ala-s-Salah," the listener should say "La hawla wa la quwwata illa billah," then when the muezzin says "Hayya 'ala-l-Falah," the listener should also say "La hawla wa la quwwata illa billah." After this, when the muezzin says "Allahu Akbar, Allahu Akbar" and "La ilaha illallah," the listener should also repeat these words. And when the listener says this with sincerity and firm belief, he will enter Paradise (provided that he also adheres to the pillars of Islam).

The conqueror of Persia, Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him), narrates from the Noble Prophet (peace and blessings be upon him) that he (peace be upon him) said: Whoever, upon hearing the call of the muezzin, says: I bear witness that there is no deity except Allah, He is One and has no partner, and Muhammad (peace and blessings be upon him) is Allah's servant and Messenger. I am pleased and content with Allah's Lordship, with Muhammad's (peace and blessings be upon him) Messengership, and I have accepted the religion of Islam—then the sins of such a person are forgiven.