Sahih Bukhari Volume 6, Book 60, Hadith Number 429.

Narrated By Musa bin ‘Uqba : ‘Abdullah bin Al-Fadl told me that Anas bin Malik said, “I was much grieve over those who had been killed in the Battle of Al-Harra. When Zaid bin Arqarr heard of my intense grief (over the killed Ansar), he wrote a letter to me saying that he heard Allah’s Apostle saying, O Allah! Forgive the Ansar and the Ansar children. The sub narrator, Ibn Al-Fadl, is not sure whether the Prophet also said, And their grand-children.” Some of those who were present, asked Anas (about Zaid). He said, “He (Zaid) is the one about whom Allah’s Apostle said, ‘He is the one whose sound hearing Allah testified.’