Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) narrates from the Messenger of Allah (peace and blessings be upon him): "Whoever, on hearing the mu'adhdhin give the adhan, says: 'Wa ana ashhadu an la ilaha illallahu wahdahu la sharika lahu wa anna Muhammadan 'abduhu wa rasuluhu, raditu billahi rabban wa bi-Muhammadin rasulan wa bil-Islami dinan' (And I too bear witness that there is no god worthy of worship but Allah, alone, without partner, and that Muhammad (peace and blessings be upon him) is His servant and His Messenger; I am pleased with Allah as Lord, with Muhammad (peace and blessings be upon him) as Messenger, and with Islam as religion), he will be forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1934