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

English translation · Sunan an-Nasa'i