Mishkat al-Masabih 2929
Muhammad b. ‘Abdallah b. Jahsh said:
When we were sitting in the courtyard of the mosque where the biers were laid down and God’s Messenger was sitting in the midst of us, he raised his eyes to the sky and looked, then lowering his eyes and putting his hand on his forehead he said, “Glory be to God, Glory be to God! What severity has come down!” We said nothing all day and night and experienced nothing but good till the morning. Then I asked God’s Messenger what the severity was which had come down, and he replied, “It has to do with debts. By Him in whose hand Muhammad’s soul is, if a man were to be killed in God’s path then come to life, be killed again in God’s path then come to life, and be killed once more in God’s path then come to life owing a debt, he would not enter paradise till his debt was paid.” 

Ahmad transmitted it, and there is something to the same effect in Sharh as-sunna .
وَعَنْ مُحَمَّدِ بْنِ عَبْدِ اللَّهِ بْنِ جَحْشٍ قَالَ: كُنَّا جُلُوسًا بِفِنَاءِ الْمَسْجِدِ حَيْثُ يُوضَعُ الْجَنَائِز وَرَسُول الله جَالِسٌ بَيْنَ ظَهْرَيْنَا فَرَفَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بَصَره قبل السَّمَاء فَنظر ثُمَّ طَأْطَأَ بَصَرَهُ وَوَضَعَ يَدَهُ عَلَى جَبْهَتِهِ قَالَ: «سُبْحَانَ الله سُبْحَانَ الله مَا نَزَلَ مِنَ التَّشْدِيدِ؟» قَالَ: فَسَكَتْنَا يَوْمَنَا وَلَيْلَتَنَا فَلَمْ نَرَ إِلَّا خَيْرًا حَتَّى أَصْبَحْنَا قَالَ مُحَمَّدٌ: فَسَأَلْتُ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: مَا التَّشْدِيدُ الَّذِي نَزَلَ؟ قَالَ: «فِي الدَّيْنِ وَالَّذِي نَفْسُ مُحَمَّدٍ بِيَدِهِ لَوْ أَنَّ رَجُلًا قُتِلَ فِي سَبِيلِ اللَّهِ ثُمَّ عَاشَ ثُمَّ قُتِلَ فِي سَبِيلِ اللَّهِ ثُمَّ عَاشَ ثُمَّ قُتِلَ فِي سَبِيلِ اللَّهِ ثُمَّ عَاشَ وَعَلَيْهِ دَيْنٌ مَا دَخَلَ الْجَنَّةَ حَتَّى يُقْضَى دَيْنُهُ» . رَوَاهُ أَحْمَدُ وَفِي شَرْحِ السُّنَّةِ نَحْوَهُ