question #1 Sahih Muslim 16c, Book 1, Hadith 21 : What are the fundamental pillars upon which Islam is built?

(The superstructure of) al-Islam is raised on five (pillars), testifying (the fact) that there is no god but Allah, that Muhammad is His bondsman and messenger, and the establishment of prayer, payment of Zakat, Pilgrimage to the House (Ka’ba) and the fast of Ramadan.