Fasting for the sake of Allah
” Every (good) deed of the son of Adam would be multiplied, a good deed receiving a tenfold to seven hundredfold reward. Allah, the Exalted and Majestic, has said: With the exception of fasting, for it is done for Me and I will give a reward for it, for one abandons his passion and food for My sake. ” [Reported by Muslim] the Messenger of Allah, sallallahu ‘alayhi wa sallam, said: “There is a gate in Paradise called Ar-Rayyan, through which only the fast observant will enter on the Day of Judgment and none except them will enter through it. It will be said: “Where are the fast observant?” They will get up, and none except them will enter through it. After their entry the gate will be closed and nobody will enter through it.”
Types of Voluntary fasting: To Fast three days every month. the white days (13-15 where the moon is full) : It is narrated on the authority of Ibn Milhan that he said: The Messenger of Allah ordered us to fast the white days of every month, the thirteenth, the fourteenth and the fifteenth, and said in that respect: “(Fasting) those days weigh (in reward fasting) the days of this life time.”
[Reported by Abu Dawud] Every Monday and Thursday: the Messenger of Allah said: “The deeds (of mankind) are presented (to Allah) every Monday and Thursday, and I like that my deed be presented (to Allah) while I am fasting.” [At-Tirmidhi]
source: http://islamicartdb.com/fasting-for-the-sake-allah/