Abu Dawud
General Behavior
Book Number: 036 / Hadith Number: 5060
Narated By Anas ibn Malik : The Prophet (pbuh) said: If anyone says in the morning: "O Allah! in the morning we call Thee, the bearers of Thy Throne, Thy angels, and all Thy creatures to witness that Thou art Allah than Whom there is no god, Thou being alone and without a partner, and that Muhammad is Thy servant and Thy Apostle," Allah will forgive him any sins that he commits that day; and if he repeats them in the evening. Allah will forgive him any sins he commits that night.