Sa’d ibn Abi Waqqas reported: The Messenger of Allah, peace and blessings be upon him, said, “Whoever says when he hears the call to prayer, ‘I testify there is no God but Allah alone, without any partner, and Muhammad is His servant and His messenger, I am pleased with Allah as a Lord, with Muhammad as a messenger, and with Islam as a religion,’ then his sins will be forgiven.”
Source: Ṣaḥīḥ Muslim 386
Grade: Sahih (authentic) according to Muslim
عَنْ سَعْدِ بْنِ أَبِي وَقَّاصٍ عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنَّهُ قَالَ مَنْ قَالَ حِينَ يَسْمَعُ الْمُؤَذِّنَ أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ رَضِيتُ بِاللَّهِ رَبًّا وَبِمُحَمَّدٍ رَسُولًا وَبِالْإِسْلَامِ دِينًا غُفِرَ لَهُ ذَنْبُهُ
386 صحيح مسلم كتاب الصلاة باب استحباب القول مثل قول المؤذن والصلاة على النبي وسؤال الوسيلة له