Musnad Ahmad 27:1555

Hashim narrated to us, Asim narrated to us, from his father, from Ibn Umar, from the Prophet, peace and blessings of Allah be upon him, 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."