As Salamu Alaikum wa Rahmatullahi wa Barakatuhu
The People Who lagged Behind:
Due to its particular circumstances, this invasion was a peculiar severe trial provided by Allah only to try the Believers’ Faith and sort them out of others. This is Allah’s permanent Will in such circumstances. In this respect He says:
“Allah will not leave the believers in the state in which you are now, until He distinguishes the wicked from the good.” [[Surah Aal-‘Imran 3]
Lagging and hanging back from full participation in that invasion amounted to the degree of hypocrisy. Whenever the Messenger of Allah (Sallallahu ‘Alayhi wa Sallam) was informed of a man’s lingering, he would say: “Leave him alone! If Allah knows him to be good He will enable him to follow you; but if he were not so, Allah would relieve us of him.”
Nobody stayed behind except those who were either hindered by a serious excuse or the hypocrites who told lies to Allah and His Messenger.
Some of those hypocrites’ lingering was due to an excuse based on forgery and delusion. Some others tarried but didn’t ask for an instant permission. But there were three believers who unjustifiably lingered. They were the ones whom Allah tried their Faith, but later on He turned to them in mercy and accepted their repentance.
As soon as the Messenger of Allah (Sallallahu ‘Alayhi wa Sallam) had entered Madinah, he prayed two Rak‘ah then he sat to receive his people. The hypocrites who were over eighty men came and offered various kinds of excuses and started swearing. The Prophet (Sallallahu 'Alayhi wa Sallam) acknowledged their excuses and invoked Allah’s forgiveness for them but he entrusted their inner thoughts and Faith to Allah.
As for the three faithful believers — Ka‘b bin Malik, Murara bin Ar-Rabi‘, and Hilal bin Omaiyah (Radhi Allahu Anhum) —who favoured telling the truth, the Messenger of Allah (Sallallahu 'Alayhi wa Sallam) bade his Companions (Radhi Allahu Anhum) not to talk to them.
Consequently they were subject to a severe boycott and were excluded from the life of the community. Everybody turned them their back. So they felt as if the whole land had become constrained to them in spite of its spaciousness and they felt awkward and uneasy. The hard times they lived and which lasted for over forty days were towered by an order to them to forsake their wives. After fifty days’ boycott Allah turned to them and revealed that in Qur’an:
“And (He did forgive also) the three [whom the Prophet (Sallallahu ‘Alayhi wa Sallam)] left (i.e. he did not give his judgement in their case, and their case was suspended for Allah’s Decision) till for them the earth, vast as it is, was straitened and their own selves were straitened to them, and they perceived that there is no fleeing from Allah, and no refuge but with Him. Then, He accepted their repentance that they might repent (unto Him). Verily, Allah is the One Who accepts repentance, Most Merciful.” [Surah At-Taubah 9]
Allah’s turning to them was a great joy for both Muslims and the three concerned. The joy of the stayers behind was unaccountable in aim and degree. It was the happiest day in their lives. The good tiding cherished them and filled their hearts with delight. As for those who lingered due to disability or sickness or any other serious excuse, Allah, the Exalted said about them:
“There is no blame on those who are weak or ill or who find no resources to spend [in holy warfare (Jihad)], if they are sincere (in duty) to Allah and His Messenger.” [Surah At-Taubah 9]
When he approached Madinah, the Messenger of Allah (Sallallahu ‘Alayhi wa Sallam) said:
“Inside Madinah, there are certain men, who though being left back due to serious excuses, they have, all the time, been with you. Lingerers as they are, they have been while you were passing valleys or walking along roads.”
“Do you mean that they have done that while they are still in Madinah?” They wondered.
“Yes though they are in Madinah,” the Prophet (Sallallahu 'Alayhi wa Sallam) said.