Abu Hurairah Radiyallahu 'anhu narrates that once Nabi Sallallahu 'alaihi wasallam asked Bilal (Radiyallahu 'anhu) at Fajr Salat:
Tell me that deed of yours
which is most hopeful (for reward)
for I heard your footsteps in front of me in Paradise
(in my dream).
Bilal (Radiyallahu 'anhu) replied:
I have not done anything extroadinary except that
whenever I perform Wudu during the day or night,
I Salat (Tahiyatul-Wudu) after that,
as much as was written or granted for me.(Bukhari)
Abu Huraira Radhiallahu ‘anhu narrated that Allah’s Messenger Shalallahu ‘alaihi wa Salam said to Bilal Radhiallahu ‘anhu “Tell me about the most hopeful act (i.e one which you deem the most rewarding with Allah) you have done since your acceptance of Islam because I heard the sound of the steps of your shoes in front of me in paradise.” Bilal said : “I do not consider any act more hopeful than that whenever I make ablution (wudoo) at any time of night or day, I offer salah (prayer) for as long as was destined for me to offer.” Narrated by Al-bukhari and Muslim.
After completion of the wudhu of two Rakaats is full of blessings (sawaab). It is related in the Hadith that performance of the two Rakaats of Tahiyatul wudhu Namaaz makes paradise obligatory (wajib) for the performer (ref:Muslim Sharif) There are also many other benefits.
It should not be performed during the Improper (makruh) times. (When the sun rises, when it is at Zenith and when it sets.)