The Book of Jihad and Expeditions
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَبْدُ الصَّمَدِ بْنُ عَبْدِ الْوَارِثِ، حَدَّثَنَا الْمُسْتَمِرُّ بْنُ الرَّيَّانِ، حَدَّثَنَا أَبُو نَضْرَةَ، عَنْ أَبِي سَعِيدٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لِكُلِّ غَادِرٍ لِوَاءٌ يَوْمَ الْقِيَامَةِ يُرْفَعُ لَهُ بِقَدْرِ غَدْرِهِ أَلاَ وَلاَ غَادِرَ أَعْظَمُ غَدْرًا مِنْ أَمِيرِ عَامَّةٍ " .It is narrated on the authority of Abu Sa'id that the Messenger of Allah (ﷺ) said:On the Day of Judgment there will be a flag for every person guilty of the breach of faith. It will be raised in proportion to the extent of his guilt; and there is no guilt of treachery more serious than the one committed by the ruler of men
From Sahih Muslim, whose reports the scholars accept as authentic.