Sahih Bukhari
Prayer at Night (Tahajjud)
Volume: 002 / Book Number: 021 / Hadith Number: 250
Narrated Abu Huraira:
At the time of the Fajr prayer the Prophet asked Bilal, "Tell me of the best deed you did after embracing Islam, for I heard your footsteps in front of me in Paradise." Bilal replied, "I did not do anything worth mentioning except that whenever I performed ablution during the day or night, I prayed after that ablution as much as was written for me."