Musnad Ahmad 30:2544

Ibn Numayr narrated to us, he said: Abu Hayyan narrated to us, from Abu Zur'ah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Bilal, tell me of the most hopeful deed you have performed in Islam that is of benefit to you, for I heard the sound of your sandals in front of me in Paradise tonight." Bilal said: "I have not performed any deed in Islam that is more hopeful to me in benefit than that I have never performed a complete ablution at any hour of the night or day, except that I prayed with that ablution what Allah had written for me to pray."