Back 
Surah 9. Al-Tawba
Ayah 43 - 45
٤٣. عَفَا ٱللَّهُ عَنكَ لِمَ أَذِنتَ لَهُمْ حَتَّىٰ يَتَبَيَّنَ لَكَ ٱلَّذِينَ صَدَقُوا۟ وَتَعْلَمَ ٱلْكَـٰذِبِينَ
٤٤. لَا يَسْتَـْٔذِنُكَ ٱلَّذِينَ يُؤْمِنُونَ بِٱللَّهِ وَٱلْيَوْمِ ٱلْـَٔاخِرِ أَن يُجَـٰهِدُوا۟ بِأَمْوَٰلِهِمْ وَأَنفُسِهِمْ ۗ وَٱللَّهُ عَلِيمٌۢ بِٱلْمُتَّقِينَ
٤٥. إِنَّمَا يَسْتَـْٔذِنُكَ ٱلَّذِينَ لَا يُؤْمِنُونَ بِٱللَّهِ وَٱلْيَوْمِ ٱلْـَٔاخِرِ وَٱرْتَابَتْ قُلُوبُهُمْ فَهُمْ فِى رَيْبِهِمْ يَتَرَدَّدُونَ
[43-45] O Prophet, may Allah forgive you: why did you give them leave to stay behind? (You yourself should not have given them leave), so that it would have become clear to you which of them spoke the truth, and you would have also known which of them were liars.45 For; those, who believe in Allah and the Last Day, will never ask you to exempt them from doing Jihad with their possessions and their persons: Allah knows well those who fear Him. Those only make such requests, who do not believe in Allah and the Last Day, and whose hearts are filled with doubts, and they are wavering because of their doubts.46
45Allah did not approve of the exemption which the Holy Prophet had granted to the hypocrites who had put forward lame excuses. Though he knew it well that they were inventing false excuses, he gave them leave to stay behind because of his inherent kindness. But Allah warned him that such a leniency was not well placed as he gave them an opportunity of hiding their hypocrisy. Had they remained behind without getting an exemption, the false profession of their faith would have been exposed.
46This verse shows clearly that the conflict between Islam and kufr is the criterion to distinguish between a true Believer and a hypocrite. The one who supports Islam in this conflict with the whole of his heart and expends all of his energies and resources in order to make it victorious and does not hesitate in the least to make any sacrifices for this purpose, is a true Believer. On the contrary, the one who hesitates to support Islam in this conflict and refrains from making sacrifices in its Way, when he confronts the threat of the victory of kufr, is a hypocrite, for such an attitude is itself a proof of the fact that there is no faith in his heart.