Translation: Don’t fast 1 or 2 days before Ramadhan, but if anybody fasts on a particular day and that day falls on these days, there is no harm. (Jame’ Tirmidhi, Vol. 1, Hadith No: 620, Pg No: 167)
The author of Majma Ul Bihar writes about the wisdom behind this prohibition:
Translation: so as to have some comfort before Ramadhan and liveliness lasts in Ramadhan as well and it is also said to separate the Nafl (optional) from the Fardh (Obligatory).
When the Holy Prophet (Sallallahu alaihi wa sallam) was asked about the most superior fasts, He (Sallallahu alaihi wa sallam) said: The fasts of Sha’abaan for respect (preparation) of Ramadhan are the best. (Ghuniya Lit talibi tariqil haqqi, Vol. 1, Pg No: 187).
Hadhrat Ghawth ul A’adham (May Allah be well pleased with him) also says in his Ghuniya Lit talibi tariqil haqqi Vol. 1, Pg No: 187:
Translation: All the sins of that person are forgiven of that person who fasts on the last Monday of Sha’abaan.
Under this Tradition, Hadhrat Ghawth ul A’adham (May Allah be well pleased with him) writes in the same book on the same page:
Translation: Which means the last Monday of Ramadhan, not the last day as welcoming Ramadhan by fasting on the last 1 or 2 days of Ramadhan is prohibited.
The Holy Prophet (Sallallahu alaihi wa sallam) would fast a lot in this month. Ummul Momineen Hadhrat Aisha Siddiqa (May Allah be well pleased with her) says:
Translation: The Holy Prophet (Sallallahu alaihi wa sallam) would fast so much in Sha’abaan that we would think, He would not have Iftaar (break His fast). Sometimes, He would not fast to the extent that we would start thinking that He will not fast now. I have never seen the Holy Prophet (Sallallahu alaihi wa sallam) fasting for the whole month except in Ramadhan and I have never seen the Holy Prophet (Sallallahu alaihi wa sallam) fasting so much in any other month as in Sha’abaan.
There is another Hadith:
Translation: It has been narrated on the authority of Hadhrat Aisha Siddiqa (May Allah be well pleased with her) that the Holy Prophet (Sallallahu alaihi wa sallam) would not fast more than Sha’abaan in any other month because in this month, the souls (Ruh) of the living are written among the dead even to the extent that a person marries when his name would taken among those who will die and verily a man would perform Haj, when his name is among the dead.
(Ibn Mardawe, Ibn ‘Asaker, Durre Manthur-Surah Dukhan)
There is another Hadith:
Translation: The Holy Prophet (Sallallahu alaihi wa sallam) would fast for the whole of Sha’abaan. When I asked Him, He (Sallallahu alaihi wa sallam) said: Allah (Subhanahu wa Ta’ala) writes (the names of) all those who would die in this year in this night. I desire that My time of passing away should be when I am fasting.
The Holy Prophet (Sallallahu alaihi wa sallam) said: This is that month between Rajab and Ramadhan, which people do not think about and this is the month in which deeds (a’amaal) are presented before Allah (Subhanahu wa Ta’ala), so I want that I should be fasting and My deeds should be presented before Allah (Subhanahu wa Ta’ala).