The Book of Jihad and Expeditions
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَعُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالاَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، حَدَّثَنَا شُعْبَةُ، عَنْ خُلَيْدٍ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لِكُلِّ غَادِرٍ لِوَاءٌ عِنْدَ اسْتِهِ يَوْمَ الْقِيَامَةِ " .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 fixed behind the buttocks of every person guilty of the breach of faith
From Sahih Muslim, whose reports the scholars accept as authentic.