Abu Uthman al-Nahdi (may Allah have mercy on him) narrates that Sayyiduna Abu Hurayrah (may Allah be pleased with him) was on a journey. When they halted, a message was sent to him to come and eat, but he was praying, so he said to the messenger: "I am fasting." But when the food was laid out and they were about to finish, he came and began to eat. The people looked at the messenger, and he said: "What are you looking at? He himself told me that he was fasting." Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: "He has spoken the truth. I heard the Messenger of Allah (peace and blessings be upon him) say: 'Fasting the month of patience and three days of every month is fasting for the whole year.' So I have fasted three days of the month; thus I am not fasting by virtue of the concession granted by Allah, and I am fasting by virtue of the multiplied reward from Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8437