Arbaeen An-Nawawi - Hadith 3

Ibn Umar reported: The Messenger of Allah, peace and blessings be upon him, said:

بُنِيَ الْإِسْلَامُ عَلَى خَمْسٍ عَلَى أَنْ يُعْبَدَ اللَّهُ وَيُكْفَرَ بِمَا دُونَهُ وَإِقَامِ الصَّلَاةِ وَإِيتَاءِ الزَّكَاةِ وَحَجِّ الْبَيْتِ وَصَوْمِ رَمَضَانَ
​
Islam is built upon five: to worship Allah and to disbelieve in what is worshiped besides him, to establish prayer, to give charity, to perform Hajj pilgrimage to the House, and to fast the month of Ramadan.
Source: Sahih Bukhari 8