Musnad Ahmad

It is narrated from Hazrat Usama bin Zaid (may Allah be pleased with him) that sometimes the Prophet Muhammad (peace and blessings be upon him) would fast so continuously that people would say, "Now the Prophet (peace and blessings be upon him) will not leave fasting," and sometimes he would leave fasting so continuously that it would seem as if he would never fast again. However, there were two days in the week that if the Prophet (peace and blessings be upon him) was fasting on them, it was very good, otherwise he would make sure to fast on them, and in no month did he observe as many voluntary fasts as he did in the month of Sha'ban. Seeing this, one day I said, "O Messenger of Allah! Sometimes you fast so much that you are not seen breaking the fast, and sometimes you leave fasting so much that you are not seen fasting. However, there are two days that if they come in your routine of fasting, it is better, otherwise you definitely fast on them." The Prophet (peace and blessings be upon him) asked, "Which two days?" I replied, "Monday and Thursday." The Prophet (peace and blessings be upon him) said, "On these two days, all deeds are presented before the Lord of the worlds, and I wish that when my deeds are presented, I am fasting." Then I said, "I do not see you observing voluntary fasts in any other month as much as I see you fasting in the month of Sha'ban." The Prophet (peace and blessings be upon him) said, "People are heedless of the importance of this month between Rajab and Ramadan, even though in this month deeds are presented before the Lord of the worlds, so I wish that when my deeds are presented, I am fasting."