I heard the messenger of Allah (ﷺ) say: Verily, al-Islam is founded on five (pillars): testifying the fact that there is no god but Allah, establishment of prayer, payment of Zakat, fast of Ramadan and Pilgrimage to the House.
Sahih Muslim Book 1, Hadith 22
← Previous Hadith (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.

Sahih Muslim Book 1, Hadith 21 Next Hadith → Messenger of Allah, verily ours is a tribe of Rabi'a and there stand between you and us the unbelievers of Mudar and we find no freedom to come to you except in the sacred month. Direct us to an act which we should ourselves perform and invite those who live beside us. Upon this the Prophet remarked: I command you to do four things and prohibit you against four acts. (The four deeds which you are commanded to do are): Faith in Allah, and then he explained it for them and said: Testifying the fact. that there is no god but Allah, that Muhammad is the messenger of Allah, performance of prayer, payment of Zakat, that you pay Khums (one-fifth) of the booty fallen to your lot, and I prohibit you to use round gourd, wine jars, wooden pots or skins for wine. Khalaf b. Hisham has made this addition in his narration: Testifying the fact that there is no god but Allah, and then he with his finger pointed out the oneness of the Lord.

Sahih Muslim Book 1, Hadith 23