Musnad Ahmad 30:1284

Muhammad ibn Bishr narrated to us, Abu Hayyan narrated to us, from Abu Zur'ah, from Abu Hurairah, who said: The Prophet of Allah, may Allah bless him and grant him peace, said to Bilal at the time of the dawn prayer: "O Bilal, inform me of the most hopeful deed you have performed in Islam, for I heard the sound of your sandals in front of me in Paradise tonight." He said: "I have not performed any deed in Islam that is more hopeful to me than that I have never performed a complete ablution at any hour of the night or day, except that I prayed with that ablution for my Lord what was written for me to pray."