Musnad Abu Ya'la al-Mawsili 155:264

Wahb narrated to us, Khalid informed us, from Abu Hayyan, from Abu Zur'ah, from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) said: "O Bilal, what is the most hopeful deed you have performed that has benefit for you in Islam?" Bilal said: "I have not performed any deed in Islam more hopeful to me, having benefit, than that I have never purified myself with an ablution during the night or the day, except that I prayed with that ablution for my Lord what was written for me to pray." He (the Prophet) said: "For indeed, I heard the sound of your sandals in front of me in Paradise tonight."