Musnad Ahmad 30:1865

'Affan narrated to us, Hammad narrated to us, Thabit narrated to us, from Abu 'Uthman, that Abu Hurairah was on a journey. When they stopped, they sent for him while he was praying, so that he might eat. He said to the messenger: "I am fasting." When the food was placed and 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? He informed me that he was fasting." Abu Hurairah said: "He spoke the truth. I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Fasting the month of patience (Ramadan) and three days of every month is fasting for a lifetime.' So I have fasted three days of every month, and I am breaking my fast according to the concession of Allah, and I am fasting according to the multiplication of Allah."