(5) باب بَيَانِ أَرْكَانِ الْإِسْلَامِ وَدَعَائِمِهِ الْعِظَامِ
It is reported on the authority of Ta'us that a man said to 'Abdullah son of 'Umar (may Allah be pleased with him). Why don't you carry out a military expedition? Upon which he replied:
I heard the messenger of Allah (ﷺ) say: Verily, al-Islam is founded on five (pillars): testifying the fact that there is no god but Allah, establishment of prayer, payment of Zakat, fast of Ramadan and Pilgrimage to the House.
Reference: Sahih Muslim 16 d
In-book reference: Book 1, Hadith 22
USC-MSA web (English) reference(deprecated numbering scheme): Book 1, Hadith 21