Musnad Abu Ya'la al-Mawsili 155:290

Bishr ibn Hilal al-Sawwaf narrated to us, he said: Abd al-Warith narrated to us, from Layth, from Mujahid, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If a man were to fast one day voluntarily, and then he were given the earth filled with gold, he would not receive his full reward except on the Day of Reckoning."