Abdullah bin Shaqiq al-Uqayli (may Allah have mercy on him) narrates: I came to Madinah and saw a tall, dark-skinned man. I said: "Who is this?" They said: "It is Sayyiduna Abu Dharr (may Allah be pleased with him)." I said: "I will certainly see what state he is in today." He says: I said: "Are you fasting?" He said: "Yes." They were waiting for permission to go in to Sayyiduna Umar (may Allah be pleased with him), and then they entered. We were brought bowls (of food), and he ate. So I nudged him, wanting to remind him with my hand, and he said: "I have not forgotten what I told you. I told you that I am fasting. I fast three days of every month, so I am always fasting."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8438