Ibn 'Umar (May Allah be pleased with them) reported

The Messenger of Allah (ﷺ) said, "(The structure of) Islam is built on five (pillars): Testification of 'La ilaha illallah' (none has the right to be worshipped but Allah), that Muhammad (ﷺ) is his slave and Messenger, the establishment of Salat, the payment of Zakat, the pilgrimage to the House of Allah (Ka'bah), and Saum during the month of Ramadan."

[Al-Bukhari and Muslim].