Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates: "My close friend (peace and blessings be upon him) advised me to do three things: to fast three days of every month, to offer the two rak'ahs of Duha, and to sleep after (praying) Witr."
(In the narration of Ibn Furak: praying Witr before sleeping, and he said: the Duha prayer.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8436
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
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