Sahih Bukhari, Chapter: Prayer at Night (Tahajjud)

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."