Hadith: Whoever says upon hearing the muezzin: 'Ash-hadu alla ilāha illallāh wahdahu la sharīka lah wa anna Muhammadan ‘abduhu wa rasūluh, radītu billāhi rabban, wa bi Muhammadin rasūlan wa bil Islāmi dīna (I bear witness that there is no god but Allah alone without any partner and that Muhammad is His slave and messenger. I am pleased with Allah as a Lord, with Muhammad as a messenger, and with Islam as a religion), his sins will be forgiven)

عَنْ سَعْدِ بْنِ أَبِي وَقَّاصٍ رضي الله عنه عَنْ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ أَنَّهُ قَالَ:
«مَنْ قَالَ حِينَ يَسْمَعُ الْمُؤَذِّنَ أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللهُ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ، رَضِيتُ بِاللهِ رَبًّا وَبِمُحَمَّدٍ رَسُولًا، وَبِالْإِسْلَامِ دِينًا، غُفِرَ لَهُ ذَنْبُهُ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 386]

Sa‘d ibn Abi Waqqās (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"Whoever says upon hearing the muezzin: 'Ash-hadu alla ilāha illallāh wahdahu la sharīka lah wa anna Muhammadan ‘abduhu wa rasūluh, radītu billāhi rabban, wa bi Muhammadin rasūlan wa bil Islāmi dīna (I bear witness that there is no god but Allah alone without any partner and that Muhammad is His slave and messenger. I am pleased with Allah as a Lord, with Muhammad as a messenger, and with Islam as a religion), his sins will be forgiven)."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 386]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informed that whoever says upon hearing the muezzin: "I bear witness that there is no god but Allah alone without any partner," i.e., I acknowledge, confess, and say that there is no deity truly worthy of worship except Allah, and all other deities are false. "And that Muhammad is His slave and messenger," i.e., he is a slave who should not be worshipped and a messenger who does not lie. "I am pleased with Allah as a Lord," i.e., with his Lordship, divinity, names, and attributes. "With Muhammad as a messenger," i.e., with everything he was sent with and conveyed to us. "And with Islam," i.e., with all the rulings of Islam including the commands and prohibitions, "as a religion," i.e., by way of belief and submission. "His sins will be forgiven," i.e., the minor sins.