From the Sunnah:

On the authority of Ibn Omar, the son of Omar ibn Al-Khattab, who said: “I heard the Messengers of Allah say: Islam has been built on five [pillars]: testifying that there is no deity worthy of worship but Allah and that Muhammad is the Messengers of Allah, performing the prayers, paying the Zakat, making the pilgrimage to the House, and fasting in Ramadan.” (Al-Bukhari and Muslim)