Sunan Ibn Majah, Chapter: The Chapters of Purification and its Sunnah

"The Messenger of Allah (saw) said: 'There is no Muslim who performs ablution and does it well, then says: Ashhadu an la ilaha illallah, wa ashhadu anna Muhammad (SAW)an 'abduhu wa rasuluhu (I bear witness that none has the right to be worshipped by Allah alone, with no partner, and I bear witness that Muhammad (SAW) is His slave and His Messenger),' (except that) eight gates of Paradise will be opened for him, and he will enter through whichever one he wants.'" (Sahih)