Allah's Messenger (ﷺ) 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 Messenger (ﷺ).
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.

Hadith

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، قَالَ أَخْبَرَنَا حَنْظَلَةُ بْنُ أَبِي سُفْيَانَ، عَنْ عِكْرِمَةَ بْنِ خَالِدٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ بُنِيَ الإِسْلاَمُ عَلَى خَمْسٍ شَهَادَةِ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ، وَإِقَامِ الصَّلاَةِ، وَإِيتَاءِ الزَّكَاةِ، وَالْحَجِّ، وَصَوْمِ رَمَضَانَ ‏"‏‏

Sahih al-Bukhari 8