Abdullah bin Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Islam has been built upon five (pillars): affirming the Oneness of Allah, establishing the prayer, paying zakah, fasting in Ramadan, and performing Hajj."
(Imam Muslim (may Allah have mercy on him) recorded it in his Sahih from Muhammad bin Abdullah bin Numayr, and added in it that a man said: Hajj and fasting in Ramadan? He said: "No, rather fasting in Ramadan and Hajj; this is how I heard it from the Messenger of Allah (peace and blessings be upon him).")
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7891