Rukun Imaan

I quote this from Sahih Bukhari, Volume 1, Book 2, Number 7:

Narrated Ibn 'Umar:

Allah's Apostle said: Islam is based on (the following) five (principles):

1. To testify that none has the right to be worshipped but Allah and Muhammad is Allah's Apostle.

2. To offer the (compulsory congregational) prayers dutifully and perfectly.

3. To pay Zakat (i.e. obligatory charity) .

4. To perform Hajj. (i.e. Pilgrimage to Mecca)

5. To observe fast during the month of Ramadan.



It's the rukun imaan. I always thought fasting on 4th, yet its on 5th.