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Authentic scholars of Tafsir say that the punishment Allah Jalla Sha'nuhu gives to a people is in proportion to their evil deed. Since these disobedient people had uttered the words: إِنَّا هَاهُنَا قَاعِدُونَ (we are sitting right here), Allah Ta'ala had them detained right there for forty years in punishment. Historical accounts in this matter differ. Some say that during these forty years, the present generation which had disobeyed was totally destroyed. Their next generation survived and it was this generation which, after having been delivered from this fortyyear detention, entered Baytul-Maqdis; or, there were some people from the earlier generation who were still surviving. However, there was that promise of the Holy Qur'an: كَتَبَ اللَّهُ لَكُمْ (that is, Allah has destined the Holy Land of Syria for the Bani Isra'il). That promise had to be fulfilled, that the Bani Isra'il should take it over and rule the land. But, the people of Ban; Isra'il present during that time elected to ignore this Divine award for which they were duly punished: مُحَرَّمَةٌ عَلَيْهِمْ ۛ أَرْبَعِينَ سَنَةً that is, they were deprived of victory over the Holy land and for forty years. Then, it was at the hands of their next generation, that this country was conquered, and the promise of Allah Ta'ala stood fulfilled.
In this wilderness of Tih, Sayyidna Musa and Harun عليه السلام were with their people for whom this wilderness was nothing but detention and punishment, while, for these two blessed souls, it was the fountain head of Divine blessings. This is why this period of forty years during which the Bani Isra'il were condemned was still full of many silver linings. Such was the barakah of the presence of Sayyidna Musa and Harun عليه السلام among them, that Allah Ta'ala showered on them blessings of many kinds. When they became helpless against the scorching sun over their heads in an open field, Allah Ta'ala, answering the prayer of Sayyidna Musa عليه السلام, stretched out a canopy of clouds over their heads. Wherever they went, these clouds would follow them with their shade. When they complained of thirst and the shortage of water, Allah Ta'ala gave Sayyidna Musa عليه السلام a special stone which went with him wherever he went, and when they needed water, Sayyidna Musa عليه السلام, would strike it with his staff and twelve streams would gush forth from it. When they were hit by hunger, the heavenly food of Mann and Salva was sent to them. And when they complained about the darkness during nights, Allah Ta'ala provided for them a tower of light under the light of which they did what they had to do.
To sum up, this wilderness of Tih was not full of condemned people only. In fact, there were two dear prophets of Allah Ta'ala and with them two favoured elders, Yusha' ibn Nun and Kalib ibn Yu'qina, as well. It was for their sake that, even during this period of detention and punishment, blessings kept coming to them. And since Allah Ta'ala is the Most Merciful of all those merciful, it is possible that these once disobedient people from the Bani Isra'il, after having witnessed the happenings around them, may have repented from their sins and may be it was in return for their act of repentance that they were being blessed with these rewards.
According to authentic narrations, it was during this period of forty years 'that Sayyidna Harun عليه السلام died. Then, after a year or six month, came the death of Sayyidna Musa عليه السلام. After them, Allah Ta'ala appointed the revered elder, Yusha' ibn Nun as a prophet and gave him the mission of guiding the Bani Isra'il. When the period of forty years of detention was over, the remaining people of Bani Isra'il got together under the leadership of Sayyidna Yusha' ibn Nun and marched towards Baytul-Maqdis on this mission of Jihad. True to the promise of Allah Ta'ala, they conquered Syria and the wealth of the land they inherited was unimaginable.
That it has been said: فَلَا تَأْسَ عَلَى الْقَوْمِ الْفَاسِقِينَ, (so do not grieve for the sinning people) at the end of verse 26 has a basis, for the blessed prophets, may peace be upon them all, are endowed with a natural disposition which makes it impossible for them to bear by the pain and hardship of their people. Even if they are punished, the prophets too are affected emotionally. Therefore, Sayyidna Musa عليه السلام has been comforted here that he should not grieve over the punishment given to those people.
Verses 27 - 32
وَاتْلُ عَلَيْهِمْ نَبَأَ ابْنَيْ آدَمَ بِالْحَقِّ إِذْ قَرَّبَا قُرْبَانًا فَتُقُبِّلَ مِنْ أَحَدِهِمَا وَلَمْ يُتَقَبَّلْ مِنَ الْآخَرِ قَالَ لَأَقْتُلَنَّكَ ۖ قَالَ إِنَّمَا يَتَقَبَّلُ اللَّهُ مِنَ الْمُتَّقِينَ
لَئِن بَسَطتَ إِلَيَّ يَدَكَ لِتَقْتُلَنِي مَا أَنَا بِبَاسِطٍ يَدِيَ إِلَيْكَ لِأَقْتُلَكَ ۖ إِنِّي أَخَافُ اللَّهَ رَبَّ الْعَالَمِينَ
إِنِّي أُرِيدُ أَن تَبُوءَ بِإِثْمِي وَإِثْمِكَ فَتَكُونَ مِنْ أَصْحَابِ النَّارِ ۚ وَذَٰلِكَ جَزَاءُ الظَّالِمِينَ
فَطَوَّعَتْ لَهُ نَفْسُهُ قَتْلَ أَخِيهِ فَقَتَلَهُ فَأَصْبَحَ مِنَ الْخَاسِرِينَ
فَبَعَثَ اللَّهُ غُرَابًا يَبْحَثُ فِي الْأَرْضِ لِيُرِيَهُ كَيْفَ يُوَارِي سَوْءَةَ أَخِيهِ ۚ قَالَ يَا وَيْلَتَا أَعَجَزْتُ أَنْ أَكُونَ مِثْلَ هَـٰذَا الْغُرَابِ فَأُوَارِيَ سَوْءَةَ أَخِي ۖ فَأَصْبَحَ مِنَ النَّادِمِينَ
مِنْ أَجْلِ ذَٰلِكَ كَتَبْنَا عَلَىٰ بَنِي إِسْرَائِيلَ أَنَّهُ مَن قَتَلَ نَفْسًا بِغَيْرِ نَفْسٍ أَوْ فَسَادٍ فِي الْأَرْضِ فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعًا وَمَنْ أَحْيَاهَا فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعًا ۚ وَلَقَدْ جَاءَتْهُمْ رُسُلُنَا بِالْبَيِّنَاتِ ثُمَّ إِنَّ كَثِيرًا مِّنْهُم بَعْدَ ذَٰلِكَ فِي الْأَرْضِ لَمُسْرِفُونَ
And recite to them the story of the two sons of Adam truthfully. When they both offered a sacrifice, then it was accepted from one of them and was not accepted from the other. He said,'I will kill you!' He said,"Allah accepts only from the God-fearing.  If you stretch your hand towards me to kill me, I am not going to stretch my hand towards you to kill you. Of course, I fear Allah, the Lord of the worlds.  I would rather have you earn my sin and your sin and then become one of the people of the Fire. And that is the punishment of the unjust!'  His self, however, prompted him to kill his brother, so he killed him and became one of the losers.  Then, Allah sent a crow who was scratching the earth to show him how he should hide the corpse of his brother. He said, "Alas!Was I not even able to be like this crow so that I could hide the corpse of my brother? So, he stood regretful. 
For this reason,We prescribed for the children of Isra'il that whoever kills a person not (to retaliate) for a person killed nor (topunish) for spreading disorder in the earth, is as if he has killed the whole mankind, and whoever saves the life of a person is as if he has saved the life of the whole mankind. And certainly, Our mes-sengers have come to them with clear signs. Then, after all that, many of them are there to commit excesses on the earth.