Musannaf Ibn Abi Shayba 27:724

Yazid ibn Harun narrated to us, from Hammad ibn Salamah, from Thabit, from Abu Bakr ibn Anas, who said: Zayd ibn Arqam wrote to Anas, consoling him for his children and his family who were afflicted on the day of al-Harrah. He wrote in his letter: "And I am bringing you glad tidings from Allah; I heard the Messenger of Allah (may Allah bless him and grant him peace) 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 children of the Ansar, and the women of the children of the children of the Ansar.'"