Musnad Ahmad Hadith 10663

Abu Uthman (may Allah have mercy on him) says that once Abu Huraira (may Allah be pleased with him) was on a journey. The people camped at a place and sent for Abu Huraira (may Allah be pleased with him) to come and eat. At that time, he was performing prayer. He sent word through the messenger that he was fasting. So the people began to eat. When they were about to finish eating, Abu Huraira (may Allah be pleased with him) also arrived and started eating. The people began to look at the messenger. He said, "Why are you staring at me? He himself told me that he was fasting." Abu Huraira (may Allah be pleased with him) said, "He is telling the truth. I heard the Noble Prophet (ﷺ) say: 'Fasting the complete month of Ramadan and three days in every month is equal to fasting the whole year.' Therefore, I fast three days every month. When I do not fast, I am under the shade of Allah's concession, and when I fast, I am under the shade of Allah's multiplication (of reward for every deed)."