Send me back, my Lord!
Allah (swt) says in Surat al-Mu'minoon, verses 99-100:
حَتَّى إِذَا جَاء أَحَدَهُمُ الْمَوْتُ قَالَ رَبِّ ارْجِعُونِ
لَعَلِّي أَعْمَلُ صَالِحًا فِيمَا تَرَكْتُ كَلَّا إِنَّهَا كَلِمَةٌ هُوَ قَائِلُهَا وَمِن وَرَائِهِم بَرْزَخٌ إِلَى يَوْمِ يُبْعَثُونَWhen death comes to one of them, he will say: "Send me back, my Lord, send me back, so that I can do good in place of what I neglected." By no means!
2- Why does Allah (swt) not accept their plea? Is it the case that in the hereafter Allah (swt) is no longer All Merciful? God forbid! Allah is always the Most Merciful.
3- Rather, the explanation for this verse lies in that those who want to go back have been detached from their bodies. In this state of detachment after death, they sincerely feel that they can reform. But let us suppose they were to be reattached to their bodies. The minute they would once again have eyes, ears, hands, etc., the temptation to sin would be too much, and they would soon revert to their old ways.
Lesson learned: our life in this world is the place and time to form the person we really want to be.