Sunan An-Nasa'i

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