Prophet (SAW)'s Prayers On hearing the Athan

Upon hearing the Athan, recite:

أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ وَأَشْهَدُ أَنَّ مُحَمَّدَاً عَبْدَهُ وَرَسُولَهُ وَأَشْهَدُ أَنَّ مُحَمَّدَاً عَبْدَهُ وَرَسُولَهُ رَضِيتُ بِاللَّهِ رَبَّاً وَبِمُحَمَّدٍ رَسُولَاً وَبِالْإِسْلَامِ دِينَاً

"I testify that there is no one worthy of worship besides Allah. He is all by Himself and has no partner and I testify that Muhammad (S.A.W.) is His servant and Rasul. I am pleased with Allah as my Lord, Muhammad (S.A.W.) as my Rasul and Islam as my Deen. It is stated in the hadith that the sins of a person are forgiven if the above dua is recited on hearing the athaan. (Muslim)