To pay Al-Khumus (one-fifth of the war booty to be given in Allah's Cause) is a part of faith

Narrated Abu Jamra: I used to sit with Ibn 'Abbas and he made me sit on his sitting place. He requested me to stay with him in order that he might give me a share from his property. So I stayed with him for two months. Once he told (me) that when the delegation of the tribe of 'Abdul Qais came to the Prophet, the Prophet (sallallahu 'alaihi wa sallam) asked them, "Who are the people (i.e. you)? (Or) who are the delegate?" They replied, "We are from the tribe of Rabi'a." Then the Prophet (sallallahu 'alaihi wa sallam) said to them, "Welcome! O people (or O delegation of 'Abdul Qais)! Neither will you have disgrace nor will you regret." They said, "O Allah's Messenger (sallallahu 'alaihi wa sallam)! We cannot come to you except in the sacred month and there is the infidel tribe of Mudar intervening between you and us. So please order us to do something good (religious deeds) so that we may inform our people whom we have left behind (at home), and that we may enter Paradise (by acting on them)." Then they asked about drinks (what is legal and what is illegal). The Prophet (sallallahu 'alaihi wa sallam) ordered them to do four things and forbade them from four things. He ordered them to believe in Allah Alone and asked them, "Do you know what is meant by believing in Allah Alone?" They replied, "Allah and His Apostle know better." Thereupon the Prophet (sallallahu 'alaihi wa sallam) said, "It means: 1. To testify that none has the right to be worshipped but Allah and Muhammad is Allah's Messenger (sallallahu 'alaihi wa sallam). 2. To offer prayers perfectly 3. To pay the Zakat (obligatory charity) 4. To observe fast during the month of Ramadan. 5. And to pay Al-Khumus (one fifth of the booty to be given in Allah's Cause). Then he forbade them four things, namely, Hantam, Dubba,' Naqir Ann Muzaffat or Muqaiyar; (These were the names of pots in which Alcoholic drinks were prepared) (The Prophet (sallallahu 'alaihi wa sallam) mentioned the container of wine and he meant the wine itself). The Prophet (sallallahu 'alaihi wa sallam) further said (to them): "Memorize them (these instructions) and convey them to the people whom you have left behind."