Jami` at-Tirmidhi, Book 49, Hadith 4277 (In-Book) ⓘ

Zaid bin Arqam wrote to Anas bin Malik, comforting him over those of his family and children of his paternal uncle who suffered on the day of Al-Harrah. So he wrote to him: "I give you glad tidings of good news from Allah. I heard the Messenger of Allah said: 'O Allah forgive the Ansar and the children of the Ansar, and the children of their children."

حَدَّثَنَا أَحْمَدُ بْنُ مَنِيعٍ، حَدَّثَنَا هُشَيْمٌ، أَخْبَرَنَا عَلِيُّ بْنُ زَيْدِ بْنِ جُدْعَانَ، حَدَّثَنَا النَّضْرُ بْنُ أَنَسٍ، عَنْ زَيْدِ بْنِ أَرْقَمَ، أَنَّهُ كَتَبَ إِلَى أَنَسِ بْنِ مَالِكٍ يُعَزِّيهِ فِيمَنْ أُصِيبَ مِنْ أَهْلِهِ وَبَنِي عَمِّهِ يَوْمَ الْحَرَّةِ فَكَتَبَ إِلَيْهِ إِنِّي أُبَشِّرُكَ بِبُشْرَى مِنَ اللَّهِ إِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏ "‏ اللَّهُمَّ اغْفِرْ لِلأَنْصَارِ وَلِذَرَارِيِّ الأَنْصَارِ وَلِذَرَارِيِّ ذَرَارِيِّهِمْ ‏" ‏ ‏.‏ قَالَ هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏