Sunan Ibn Majah Book 3, Hadith 721

حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ الْمِصْرِيُّ، أَنْبَأَنَا اللَّيْثُ بْنُ سَعْدٍ، عَنِ الْحُكَيْمِ بْنِ عَبْدِ اللَّهِ بْنِ قَيْسٍ، عَنْ عَامِرِ بْنِ سَعْدِ بْنِ أَبِي وَقَّاصٍ، عَنْ سَعْدِ بْنِ أَبِي وَقَّاصٍ، عَنْ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ أَنَّهُ قَالَ ‏ "‏ مَنْ قَالَ حِينَ يَسْمَعُ الْمُؤَذِّنَ وَأَنَا أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ رَضِيتُ بِاللَّهِ رَبًّا وَبِالإِسْلاَمِ دِينًا وَبِمُحَمَّدٍ نَبِيًّا غُفِرَ لَهُ ذَنْبُهُ ‏"‏ ‏.‏

It was narrated from Sa'd bin Abu Waqqas that:The Messenger of Allah said: "Whoever says, when he hears the Mu'adh-dhin, 'Wa ana Ash-hadu an la ilaha illallah wahdahu la sharika lahu, wa ash-hadu anna Muhammadan 'abduhu wa rasuluhu, radaytu Billahi rabban wa bil-islami dinan wa bi muhammadin nabiyyan (And I bear witness that none has the right to be worshipped but Allah alone, with no partner, and I bear witness that Muhammad is His slave and Messenger, and I am content with Allah as my Lord, Islam as my religion and Muhammad as my Prophet),' his sins will be forgiven to him