Authentic Supplication Number 044.

It is reported by way of 'Abi Sa`eed  that the Prophet  said: "Whoever performs wudhu' then says after he has completed his wudhu': Subhanaka 'allahumma wa bihamdika, 'ash-hadu 'an la 'ilaha 'illa 'anta, 'astaghfiruka wa 'atoobu 'ilayka (66), it would be written on vellum then sealed; thereafter, it would not be broken till The Day of Resurrection".

Footnotes.
(66) Glorified are You O' my 'ilah and I am in Your praise, I testify that there is no 'ilah except You, I ask Your forgiveness and repent unto You.

Reporters.
Al-Albani said it was sound in Saheeh 'Al-Jami` as-Sagheer wa Ziyadatih and that it was reported by An-Nisa'ee, and Al-Hakim.