Musnad Ahmad 27:1841

Ibn Numayr narrated to us, Hanzalah narrated to us, I heard ‘Ikrimah ibn Khalid narrating to Tawus, he said: A man said to ‘Abdullah ibn ‘Umar: "Will you not go on a military expedition (ghazw)?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Indeed, Islam is built upon five: the testimony that there is no god but Allah, the establishment of prayer, the giving of zakat, the fasting of Ramadan, and the pilgrimage to the House.'"