Musnad Ahmad 983
It was narrated that ‘Abeedah said:
When `Ali killed the people of an-Nahrawan he said: Look for him. And they found him in a ditch lying beneath the slain. They brought him out and ‘Ali               (رضي الله عنه) came to his companions and said: If you could exercise restraint, I would have told you what Allah promised on the lips of Muhammad to those who kill them. I said: Did you hear that from the Messenger of Allah (ﷺ)? He said: Yes, by the Lord of the Ka`bah.
حَدَّثَنَا عَبْد اللَّهِ، حَدَّثَنِي مُحَمَّدُ بْنُ أَبِي بَكْرٍ الْمُقَدَّمِيُّ، حَدَّثَنَا حَمَّادُ بْنُ يَحْيَى الْأَبَحُّ، حَدَّثَنَا ابْنُ عَوْنٍ، عَنْ مُحَمَّدٍ، عَنْ عَبِيدَةَ، قَالَ لَمَّا قَتَلَ عَلِيٌّ أَهْلَ النَّهْرَوَانِ قَالَ الْتَمِسُوهُ فَوَجَدُوهُ فِي حُفْرَةٍ تَحْتَ الْقَتْلَى فَاسْتَخْرَجُوهُ وَأَقْبَلَ عَلِيٌّ رَضِيَ اللَّهُ عَنْهُ عَلَى أَصْحَابِهِ فَقَالَ لَوْلَا أَنْ تَبْطَرُوا لَأَخْبَرْتُكُمْ مَا وَعَدَ اللَّهُ مَنْ يَقْتُلُ هَؤُلَاءِ عَلَى لِسَانِ مُحَمَّدٍ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قُلْتُ أَنْتَ سَمِعْتَهُ مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ إِي وَرَبِّ الْكَعْبَةِ‏.‏