Chapter 47: The Book Of Faith and its Signs
It was narrated from Ibn 'Umar that: A man said to him: "Why don' t you go out and fight?" He said: "I heard the Messenger of Allah [SAW] say: 'Islam is built on five (pillars): Testimony that there is none worthy of worship except Allah, establishing Salah, giving Zakah, Hajj, and fasting Ramadan.'"
Sunan an-Nasa'i 5001