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

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."

Reference: Muslim: 386