The payment of Khumus is a part of religion

Narrated Ibn 'Abbas:

The delegates of the tribe of 'Abdul-Qais came and said, "O Allah's Apostle! We are from the tribe of Rabi'a, and there is the infidels of the tribe of Mudar intervening between you and us, so we cannot come to you except in the Sacred Months. So please order us some instructions that we may apply it to ourselves and also invite our people whom we left behind us to observe as well." The Prophet ﷺ said, "I order you (to do) four (things) and forbid you (to do) four: I order you to believe in Allah, that is, to testify that None has the right to be worshipped but Allah (the Prophet pointed with his hand),
to offer prayers perfectly;
to pay Zakat;
to fast the month of Ramadan, and to pay the Khumus (i.e. one-fifth) of the war booty to Allah and I forbid you to use Ad-dubba', An-Naqir, Al-Hantam and Al-Muzaffat (i.e. utensils used for preparing alcoholic drinks)." (See Hadith No. 50, Vol. 1).

حَدَّثَنَا أَبُو النُّعْمَانِ، حَدَّثَنَا حَمَّادٌ، عَنْ أَبِي حَمْزَةَ الضُّبَعِيِّ، قَالَ سَمِعْتُ ابْنَ عَبَّاسٍ ـ رضى الله عنهما ـ يَقُولُ قَدِمَ وَفْدُ عَبْدِ الْقَيْسِ فَقَالُوا يَا رَسُولَ اللَّهِ، إِنَّا هَذَا الْحَىَّ مِنْ رَبِيعَةَ، بَيْنَنَا وَبَيْنَكَ كُفَّارُ مُضَرَ، فَلَسْنَا نَصِلُ إِلَيْكَ إِلاَّ فِي الشَّهْرِ الْحَرَامِ، فَمُرْنَا بِأَمْرٍ نَأْخُذُ مِنْهُ وَنَدْعُو إِلَيْهِ مَنْ وَرَاءَنَا. قَالَ " آمُرُكُمْ بِأَرْبَعٍ، وَأَنْهَاكُمْ عَنْ أَرْبَعٍ، الإِيمَانِ بِاللَّهِ شَهَادَةِ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ ـ وَعَقَدَ بِيَدِهِ ـ وَإِقَامِ الصَّلاَةِ وَإِيتَاءِ الزَّكَاةِ وَصِيَامِ رَمَضَانَ، وَأَنْ تُؤَدُّوا لِلَّهِ خُمُسَ مَا غَنِمْتُمْ، وَأَنْهَاكُمْ عَنِ الدُّبَّاءِ وَالنَّقِيرِ وَالْحَنْتَمِ وَالْمُزَفَّتِ ".