After uttering Mu'adhdhin Tashahhud (i.e. Ashhadu Anna Muhammadar ...) the listeners will say this dhikr -
أَشْهَدُ أَنْ لَا إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ رَضِيتُ بِاللَّهِ رَبًّا وَبِمُحَمَّدٍ رَسُولًا وَبِالْإِسْلَامِ دِينًا
اَشْهَدُ اَنْ لَّا اِلٰهَ اِلَّا اللّٰهُ وَحْدَهٗ لَا شَرِيْكَ لَهٗ وَاَنَّ مُحَمَّدًا عَبْدُهٗ وَرَسُوْلُهٗ ، رَضِيْتُ بِاللّٰهِ رَبًّا وَّبِمُحَمَّدٍ رَّسُوْلًا وَّبِالْاِسْلَامِ دِيْنًا
Ash-hadu an laa ilaaha ill-Allahu wa'hdahu laa shareeka lahu wa anna mu'hammadan ‛abduhu wa rasooluhu, radeetu billaahi rabban wa bimu'hammadin rasoolan wa bil -islaami deen-an
I bear witness that none has the right to be worshipped except Allah, alone, without partner, and that Muhammad is His slave and Messenger (ﷺ). I am pleased with Allah as a Lord, and Muhammad as a Messenger and Islam as a religion.
Sa'd ibn Abi Waqqas (RA) said: The Messenger of Allah (ﷺ) said: "Whoever hears Mu'adhzin and utters these words, all his sins will be forgiven."