Once food was presented to Sayyiduna Umar (may Allah be pleased with him), he invited a man to join him. The man said, "I am fasting." Umar asked, "Which fast are you observing?" He said, "If there was no fear of error, I would narrate to you the hadith of the Prophet (peace and blessings be upon him) when a Bedouin brought a rabbit to the Prophet (peace and blessings be upon him)." He said, "Go and call Sayyiduna Ammar (may Allah be pleased with him)." When Sayyiduna Ammar (may Allah be pleased with him) arrived, Sayyiduna Umar (may Allah be pleased with him) asked him, "Were you present with the Prophet (peace and blessings be upon him) on the day when a Bedouin brought a rabbit?" He replied, "Yes! I saw blood on it." The Prophet (peace and blessings be upon him) said, "Eat it." The man said, "I am fasting." The Prophet (peace and blessings be upon him) asked, "What kind of fast?" He said, "I fast at the beginning and end of every month." The Prophet (peace and blessings be upon him) said, "If you want to fast, then for voluntary fasts, choose the 13th, 14th, and 15th days of the month."