(4) باب مَا يُقَالُ إِذَا أَذَّنَ الْمُؤَذِّنُ
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."
Reference: Sunan Ibn Majah 721
In-book reference: Book 3, Hadith 16
English translation: Vol. 1, Book 3, Hadith 721