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

that the Prophet (saw) said: "Whoever performs ablution and does it well, then says three times: 'Ashhadu an la ilaha illallah wahdahu la sharika lahu, wa ashshadu anna Muhammad (SAW)an 'abduhu wa rasuluhu (I bear witness that none has the right to be worshipped but Allah alone, with no partner, and I bear witness that Muhammad (SAW) is His slave and His Messenger),' eight gates of Paradise will be opened for him; whichever one he wants he may enter." (Daif) Another chain with similar wording.