Musnad Ahmad 30:3532

Rawh narrated to us, Hammad narrated to us, from Thabit, from Abu Uthman, that Abu Hurayrah was on a journey. When they stopped, they sent for him while he was praying. He said: "I am fasting." When they placed the food and were about to finish, he came. They said: "Come and eat." So he ate. The people looked at the messenger [who had called him], and he said: "What are you looking at?" He said: "By Allah, he said: 'I am fasting.'" Abu Hurayrah said: "He spoke the truth. Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) said: 'Fasting the month of patience [Ramadan] and three days of every month is the fasting of the entire lifetime.' I have fasted three days from the beginning of the month, so I am breaking the fast according to the concession of Allah, and I am fasting according to the multiplication of Allah."