´It was narrated that 'Alqamah bin Waqqas said:` "I was with Mu'awiyah when the Mu'adhdhin called the Adhan. Muawiyah said what the Mu'adhdhin said, but when he said: 'Hayya 'alas-salah (come to prayer),' he said: 'La hawla wa la quwwata illa Billah (There is no power and no strength except with Allah);' and when he said: 'Hayya 'alal-falah (come to prosperity),' he said: 'La hawla wa la quwwata illa Billah (There is no power and no strength except with Allah).' After that he said what the Mu'adhdhin said, then he said: 'I heard the Messenger of Allah (S.A.W) saying exactly like that.'"
Narrated Abu Umama bin Sahl bin Hunaif: I heard Muawiya bin Abi Sufyan (repeating the statements of the Adhan) while he was sitting on the pulpit. When the Mu'adh-dhin pronounced the Adhan saying, "Allahu-Akbar, Allahu Akbar", Muawiya said: "Allah Akbar, Allahu Akbar." And when the Mu'adh-dhin said, "Ash-hadu an la ilaha illal-lah (I testify that none has the right to be worshipped but Allah)", Muawiya said, "And (so do) I". When he said, "Ash-hadu anna Muhammadan Rasulullah" (I testify that Muhammad is Allah's Apostle), Muawiya said, "And (so do) I". When the Adhan was finished, Muawiya said, "O people, when the Mu'adh-dhin pronounced the Adhan I heard Allah's Apostle on this very pulpit saying what you have just heard me saying".
Shaykh Abdul Sattar al-Hammad
Hadith Commentary: From this hadith, the recommendation (istihbab) for the imam and the khatib to respond to the adhan is understood; other people should also respond to the adhan. It is also evident that even if the imam is seated on the pulpit (minbar), the process of teaching and learning may continue.
In addition, the response to the two testimonies (shahadatayn) can also be given in the first person pronoun. Accordingly, Imam Ibn Hibban rahimahullah has established a separate chapter regarding this in his Sahih. (: Sahih Ibn Hibban: 97/4)
Source: Hidayat al-Qari: Commentary on Sahih Bukhari, Urdu, Page: 914