Musnad Ahmad 941:12

'Abd al-Rahman ibn Mahdi narrated to us, Thabit ibn Qays Abu Ghusn narrated to us, Abu Sa'id al-Maqburi narrated to me, Usama ibn Zayd narrated to me, he said: The Messenger of Allah (peace and blessings of Allah be upon him) used to fast days consecutively until it would be said: "He does not break his fast," and he would break his fast for days until he would almost not fast except for two days of the week, if they fell within his fasting, otherwise he would fast them. And he would not fast in any month as much as he fasted in Sha'ban. I said: "O Messenger of Allah, you fast until you almost do not break your fast, and you break your fast until you almost do not fast except for two days, if they fall within your fasting, otherwise you fast them." He said: "Which two days?" I said: "Monday and Thursday." He said: "Those are two days in which deeds are presented to the Lord of the Worlds, and I love for my deeds to be presented while I am fasting." He said: "I said: And I have not seen you fast in any month as much as you fast in Sha'ban." He said: "That is a month people are heedless of, between Rajab and Ramadan, and it is a month in which deeds are raised to the Lord of the Worlds, so I love for my deeds to be raised while I am fasting."