Ayesha said: "I never saw the Messenger of Allah fast a complete month except for Ramadan, and I have never seen him fast more in a month than he did in Shaban." [Bukhaari, Muslim]
Usamah ibn Zaid inquired: "O Messenger of Allah, I never find you fasting in any month like you do during the month of Shaban." The Prophet responded: "That is the month the people neglect. It comes between Rajab and Ramadan. It is a month in which the deeds are raised to the Lord of the Worlds. I love that my deeds be raised while I am fasting." [An-Nasaa'i, authenticated by ibn Khuzaimah. Hasan according to al-Albaani]
Narrated Abu Hurairah, Allah's Messenger said: "When the month of Shaban is halfway, do not fast." [Reported by the five, Ahmad disapproved it].
As-San`aanee says in Subul us-Salaam: "One can fast provided he has to observe a compensatory fasting (in lieu of the Fard - obligatory ones) or the other prescribed ones which may be categorised as Wajib (compulsory). However, the volitional fasting is prohibited lest one should be subjected to exhaustion and weakness that may in turn render the Ramadan obligatory fasting difficult for him."