Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 749
Sa'd b. Abu Waqqas reported: The Messenger of Allah (may peace be upon him) said: If anyone says on hearing the Mu'adhdhin: I testify that there is no god but Allah alone. Who has no partner, and that Muhammad is His servant and His Messenger, (and that) I am satisfied with Allah as my Lord, with Muhammad as Messenger. and with Islam as din (code of life), his sins would be forgiven. In the narration transmitted by Ibn Rumh the words are:" He who said on hearing the Mu'adhdhin and verity I testify." ' Qutaiba has not mentioned his words:" And I."