Musnad Abu Ya'la al-Mawsili 161:2

'Abd al-A'la narrated to us, Hammad narrated to us, from Thabit, from Abu 'Uthman, that Abu Hurayrah was on a journey. When they stopped and the dining cloth was laid out, they sent for him while he was praying. He said, "I am fasting." When they were about to finish, he came and began to eat. The people looked at their messenger, and he said, "What are you looking at? By Allah, he informed me that he was fasting." Abu Hurayrah said, "He spoke the truth. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Whoever fasts three days of every month has fasted the entire lifetime.' And I have fasted three days of every month, so it is as if I have fasted the whole month. I found the confirmation of that in the Book of Allah, the Mighty and Majestic: 'Whoever comes with a good deed, he shall have ten times the like thereof.'" He recited it again, then said, "I have fasted three days from the beginning of the month, and I am breaking my fast in the alleviation of Allah, yet fasting in the multiplication of Allah, the Mighty and Majestic."