Sahih Muslim Book 35, Hadith Number 6514.
Chapter : The merit of pronouncing Tablil, Tasbib and supplication.
Abu Malik Ashaja’i reported on the authority of his father that whenever a person embraced Islam, Allah’s Messenger (may peace be upon him) instructed him to recite: “O Allah, grant me pardon, have mercy upon me, direct me to the path of righteousness and provide me sustenance.”