Authentic Supplication Number 045.

It is reported by way of 'Umar ibn Al-Khattab that the Prophet  said: "There is not one of you that makes wudhu', and performs the rites of wudhu', then says when he has completed his wudhu': 'Ash-hadu 'an la 'ilaha 'illa 'allahu wahdahu la shareeka lahu, wa 'anna muhammadan `abduhu wa rasooluhu (67), except that the eight gates of Heaven would open up for him, that he may enter from whichever he would please".

Footnotes.
(67) I bear witness that there is no 'ilah except Allah alone, no partner has he, and that Muhammad is His bondservant, and Messenger.

Reporters.
Reported by Muslim, Abu Dawood, An-Nisa'ee and Ahmad. Al-Albani said it was sound in his Saheeh 'Al-Jami` As-Sagheer wa Ziyadatih. This hadeeth is #162 in his Saheeh 'Abi-Dawood, and #96 in 'Al-'irwa'.