Al-Haafidh Ibn Rajab said in his essay entitled Kalimatul-Ikhlaas (pp.20-21), whilst commenting upon the saying of the Prophet sallallahu alaihi wasallam: I have been commanded to fight the people, until they testify to laa ilaaha illallaah and that I am the Messenger of Allaah, and establish the Prayer and pay the Zakaah.”(1).
He said: “That ‘Umar and a group of the Companions understood [from it] that whoever pronounces the shahaadatain, would be protected from being punished in this world because of it. So they withheld from fighting against those who withheld from paying the Zakaah. However, [Abu Bakr] as-Siddeeq understood [from it] that fighting them should not be prevented, unless they fulfilled the rights of the kalimah, due to the Prophet sallallaahu ‘alayhi wa sallam’s saying:
“If they do this, then their blood and their property will be protected from me, except what is a right by Islaam, and their reckoning is with Allaah.”(2).
Abu Bakr said: “Zakaah is a right which is due upon wealth.”(3)
This understanding of Abu Bakr as-Siddeeq was based upon what was clearly narrated from the Prophet sallallahu alaihi wasallam by more than one Companion - which includes Ibn ‘Umar, Anas and others - that he said:
“I have been commanded to fight the people, until they testify to laa ilaaha illallaah and that I am the Messenger of Allaah, and establish the Prayer and give the Zakaah.”
This is also proven by Allaah the Most High’s saying:
“So if they repent (from their unbelief), establish the Prayer and give the Zakaah, then leave their way free.” [Soorah at-Tawbah 9:5]
In a similar way, the [following] verse: “So if they repent, establish the Prayer and give the Zakaah, then they are your brothers in the Religion.” [Soorah at-Tawbah 9:11] … proves that the Islaamic brotherhood is not established except by fulfillment of the obligatory duties along with [the affirmation of] tawheed. Indeed, repentance from shirk can never be achieved except with the [affirmation of] tawheed. So when Abu Bakr resolved this with the Companions, they accepted his saying and saw that his view was correct. So now you know that punishment in this world is not absolutely lifted from one who merely utters the two testimonies of faith, rather he may receive punishment due to certain shortcomings in fulfilling any of the rights of Islaam. Likewise, there could [also] be punishment in the Hereafter.”
Ibn Rajab also said: “A group of scholars said that what is meant by these ahaadeeth is that laa ilaaha illallaah is a cause for entering the Gardens of Paradise and being saved from the Hellfire, and that it obligates it. However, results will not come about except by the fulfillment of its conditions (shuroot) and the removal of any of its impediments (intifaa’ul-muwaani’).
Consequently, rewards may be withheld due to the absence of one of its conditions, or due to the presence of one of its impediments. This is the saying of al-Hasan [al-Basree] and Wahb ibn Munabbih and it is the most prevalent view.
(1) Reported by al-Bukhaaree (no.25) and Muslim (no.36), from Ibn ‘Umar.
(2) This is the end part of the preceding hadeeth.
(3) Reported by al-Bukhaaree (no.1400).