The three months of continuous fasting

I first came across this a few years back when a friend of mine did it. He will start his fasting from the month of Rejab, right through Shaaban and finally Ramadhan. That means three months of continuous fasting. Even at that time, I asked myself, where did he get this practise from. Certainly not from the prophet pbuh. However, I kept the question to myself, not wanting to offend him. But lately, I found that this practise is spreading. I met other friends who do this. Really is a surprise because I'm pretty sure this is not the practise of the prophet pbuh.