Musnad Abu Ya'la al-Mawsili 154:158

Abu Khaythama narrated to us, Hashim ibn al-Qasim narrated to us, 'Asim narrated to us, from his father, from Ibn 'Umar, from the Prophet (may Allah bless him and grant him peace), who said: "Islam is built upon five: the testimony that there is no god but Allah and that Muhammad is the Messenger of Allah, the establishment of prayer, the giving of zakat, the pilgrimage to the House, and the fasting of Ramadan."