Shama-il Tirmidhi Chapter 042, Hadith Number 005 (285).

"Aayeshah Radiyallahu 'Anha says: "I did not observe Rasulullah Sallallahu 'Alayhi Wasallam fast for more days in any month (excluding Ramadaan) other than Sha'baan. He fasted for the major part of the month, and nearly fasted for the full month".

Commentary
This goes a bit further where Sayyiditina 'Aayeshah Radiyallahu 'Anha clearly mentions the fasting of the full month of Sha'baan, by this, an exaggeration is indicated. The reason for fasting the major portion of Sha'baan is mentioned by Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam himself that, 'In this month, there is also that day wherein the deeds of the year are presented before Allah. I love that my deeds should be presented whilst I am fasting'. Besides this many other reasons are narrated in the ahaadith. It is possible that at a certain time it is for a particular reason and at another time it is for some other reason. The combining of many reasons at one time has also been mentioned by Sayyiditina 'Aayeshah Radiyallahu 'Anha, that the Practice of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam was to fast for three days in every month. At times, due to unforeseen circumstances these were not observed. The total qadaa (of missed fasts) were combined and kept in Sha'baan by Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam. In another narration it is mentioned that it was the practice of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam to fast on every Monday and Thursday. In this manner during the course of the year, due to circumstances the fasts of two or three months could not be observed. It could be possible for these to add up to a full month. In one narration it has been stated that because of the sanctity and greatness of Ramadaan the fasts of Sha'baan are meritorious. i.e. In the manner that sunan are performed before the fard salaah, likewise there are nafl fasts before Ramadaan. Weakness due to fasting did not affect Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam, therefore at times he fasted for consecutive days. It is stated in the hadith that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam was once asked the reason for fasting so many days of Sha'baan. He replied: 'The names of all those who are going to die during the course of the year, are written down in this month. I desire that my death be written in a state when I am fasting'. Some of the 'ulama have written that it was a special practice of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam to observe nafl saum in every month, and there was no way to observe nafl saum in the month of Ramadaan, therefore those nafl portions of Ramadhaan were kept in advance in the month of Sha'baan. For this reason if the fasts of both Sha'baan and Ramadaan are combined, it will cover most of the month.