Tawus reported: A man said to Abdullah ibn Umar, “Why do you not join the military campaigns?” Ibn Umar said, “I heard the Messenger of Allah, peace and blessings be upon him, say: Verily, Islam is built upon five: to testify there is no God but Allah, to establish prayer, to give charity, to fast the month of Ramadan, and to perform pilgrimage to the House.”
Source: Ṣaḥīḥ Muslim 16
Grade: Sahih (authentic) according to Muslim
عَنْ طَاوُسً أَنَّ رَجُلًا قَالَ لِعَبْدِ اللَّهِ بْنِ عُمَرَ أَلَا تَغْزُو فَقَالَ إِنِّي سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ إِنَّ الْإِسْلَامَ بُنِيَ عَلَى خَمْسٍ شَهَادَةِ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَإِقَامِ الصَّلَاةِ وَإِيتَاءِ الزَّكَاةِ وَصِيَامِ رَمَضَانَ وَحَجِّ الْبَيْتِ
16 صحيح مسلم كتاب الإيمان باب بيان أركان الإسلام ودعائمه العظام