Why do Muslims fast during the month of Ramadan ?

O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous –

[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] – then an equal number of other days [are to be made up]. And upon those who are able [to fast, but with hardship] – a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers good [i.e., excess] – it is better for him. But to fast is best for you, if you only knew.
Quran ( 2 : 183 - 184 )