It is narrated from Nadr bin Anas that Sayyiduna Zaid bin Arqam (may Allah be pleased with him) wrote to Sayyiduna Anas bin Malik (may Allah be pleased with him) during the days of the incident of Harrah to console him over the killing of his children and members of his tribe, and said: I want to give you glad tidings from Allah Almighty. I heard the Messenger of Allah (peace and blessings be upon him) say: O Allah! Forgive the Ansar, their sons, and their grandsons. Forgive the women of the Ansar, the wives of the sons of the Ansar, and the women of the grandsons of the Ansar.
Hadith Referenceالفتح الربانی / كتاب الفضائل والمناقب / 11538
Benefits: … Yazid ibn Muawiyah, during his reign, sent an army to attack Madinah; this is known as the incident of Harrah.
The context of the narration in Sahih Bukhari is as follows: From Abdullah ibn al-Fadl, that he heard Anas ibn Malik say: "I was grieved over those who were afflicted in Harrah. So Zayd ibn Arqam wrote to me, and when he learned of the intensity of my grief, he mentioned that he had heard the Messenger of Allah (sallallahu alayhi wa sallam) say: 'O Allah, forgive the Ansar and the sons of the Ansar.'"
… It is narrated from our master Anas ibn Malik (radi Allahu anhu), he says: I was saddened over those who were martyred in Harrah. When our master Zayd ibn Arqam (radi Allahu anhu) learned of the intensity of my grief, he wrote to me that the Messenger of Allah (sallallahu alayhi wa sallam) said: O Allah! Forgive the Ansar and the sons of the Ansar.