Hadees, + -

You who believe! Fasting is prescribed for you, as it was prescribed
for those before you - so that hopefully, you will fear [and respect]
Allah. (Surat al-Baqara, 183)
"Islam is based on five things: the testimony that there is no deity
but Allah and that Muhammad (saas) is His servant and messenger, the
observance of the prayer, the payment of Zakat, the pilgrimage, and
the fast during Ramadan. " (Bukhari, Muslim)