Musnad Ahmad 771:81

Yazid narrated to us, Hammad ibn Salamah informed us, from Ali ibn Zayd, from Abu Bakr ibn Anas, who said: Zayd ibn Arqam wrote to Anas ibn Malik, consoling him for those who were afflicted among his children and his people on the day of al-Harrah. So he wrote back to him: "And I bring you glad tidings from Allah, the Almighty and Majestic. I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'O Allah, forgive the Ansar, and the children of the Ansar, and the children of the children of the Ansar, and the women of the Ansar, and the women of the children of the Ansar, and the women of the children of the children of the Ansar.'"