Riyad as-Salihin

Ibn 'Umar (May Allah be pleased with them) reported: The Messenger of Allah (ﷺ) said, "Islam is based on five (pillars): testifying that there is no true god except Allah and that Muhammad (ﷺ) is His slave and Messenger; performing of Salat (Iqamat-as-Salat); the payment of Zakat; performing Hajj (pilgrimage) to the House [of Allah (Ka'bah)]; and Saum (fasting) during the month of Ramadan."

[Al-Bukhari and Muslim].