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 (ﷺ) said: 'O Allah forgive the Ansar and the children of the Ansar, and the children of their children." (JA)
English reference (JA): Vol. 1, Book 46, Hadith 3902 (JA)
Arabic reference (JA): Book 49, Hadith 4277 (JA)