Sunan Ibn Majah Book 1, Hadith 203
The Prophet said: "Whoever performs ablution and does it well, then says three times: 'Ashhadu an la ilaha illallah wahdahu la sharika lahu, wa ashhadu anna Muhammadan `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 is His slave and His Messenger),' eight gates of Paradise will be opened for him; whichever one he wants he may enter." (Da'if)
Another chain with similar wording.
The Messenger of Allah performed ablution, then he turned inside out the woolen garment that he was wearing and wiped his face with it.
"The Messenger of Allah said: 'There is no Muslim who performs ablution and does it well, then says: Ashhadu an la ilaha illallah, wa ashhadu anna Muhammadan `abduhu wa rasuluhu (I bear witness that none has the right to be worshipped but Allah, and I bear witness that Muhammad is His slave and Messenger),' (except that) eight gates of Paradise will be opened for him, and he will enter through whichever one he wants.'"