It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When you hear the adhan, say the same as the mu'adhdhin says."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1925
It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the mu'adhdhin says: 'Allahu akbar, Allahu akbar' (Allah is the Greatest, Allah is the Greatest), you too say: 'Allahu akbar, Allahu akbar'. Then he says: 'Ashhadu an la ilaha illallah' (I bear witness that there is no god but Allah), and you say: 'Ashhadu an la ilaha illallah'. Then he says: 'Ashhadu anna Muhammadan rasulullah' (I bear witness that Muhammad is the Messenger of Allah), and you say: 'Ashhadu anna Muhammadan rasulullah'. Then he says: 'Hayya 'ala al-salah' (Come to the prayer), and you say: 'La hawla wa la quwwata illa billah' (There is no might and no power except with Allah). Then he says: 'Hayya 'ala al-falah' (Come to success), and you say: 'La hawla wa la quwwata illa billah'. Then he says: 'Allahu akbar, Allahu akbar', and you too say: 'Allahu akbar, Allahu akbar'. Then he says: 'La ilaha illallah' (There is no god but Allah); and whoever says 'La ilaha illallah' from his heart will enter Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1926
Abu Umamah bin Sahl bin Hunayf said: I heard Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) while he was sitting on the pulpit; the mu'adhdhin gave the adhan and said: 'Allahu akbar, Allahu akbar' (Allah is the Greatest, Allah is the Greatest). Sayyiduna Mu'awiyah (may Allah be pleased with him) said: "Allahu akbar, Allahu akbar." He said: 'Ashhadu an la ilaha illallah' (I bear witness that there is no god but Allah), and Sayyiduna Mu'awiyah (may Allah be pleased with him) said the like of it. He said: 'Ashhadu anna Muhammadan rasulullah' (I bear witness that Muhammad is the Messenger of Allah), and Sayyiduna Mu'awiyah (may Allah be pleased with him) also said: "And I too." Then when he completed the adhan, he said: O people, I heard the Messenger of Allah (peace and blessings be upon him) on this very seat, when the mu'adhdhin gave the adhan, say the same as what you have heard from me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1927
Isa bin Talhah (may Allah have mercy on him) said: We went in to Sayyiduna Mu'awiyah (may Allah be pleased with him). The mu'adhdhin gave the adhan and said: 'Allahu akbar, Allahu akbar' (Allah is the Greatest, Allah is the Greatest), and Sayyiduna Mu'awiyah (may Allah be pleased with him) said: "Allahu akbar, Allahu akbar." He said: 'Ashhadu an la ilaha illallah' (I bear witness that there is no god but Allah), and Sayyiduna Mu'awiyah (may Allah be pleased with him) said: "Ashhadu an la ilaha illallah." He said: 'Ashhadu anna Muhammadan rasulullah' (I bear witness that Muhammad is the Messenger of Allah), and Sayyiduna Mu'awiyah (may Allah be pleased with him) said: "And I too: Ashhadu anna Muhammadan rasulullah." Yahya said: A companion of ours narrated to us that when he said: 'Hayya 'ala al-salah' (Come to the prayer), he said: 'La hawla wa la quwwata illa billah' (There is no might and no power except with Allah), then he said: "This is how we heard it from your Prophet (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1928
It is narrated from Sayyidah Aishah (may Allah be pleased with her) that when the Messenger of Allah (peace and blessings be upon him) heard the mu'adhdhin bearing witness, he (peace and blessings be upon him) would say: "And I too, and I too (bear witness)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1929