On the authority of Abdullah, the son of Umar ibn al-Khattab (may Allah be pleased with him), who said, "I heard the Messenger of Allah (ﷺ) say, 'Islam has been built on five [pillars]: Testifying that there is no deity worthy of worship except Allah and that Muhammad is the Messenger of Allah, establishing the salah (prayer), paying the zakat (obligatory charity), making the hajj (pilgrimage) to the House, and fasting in Ramadhan.'" [Bukhari & Muslim]