Mustadrak al-Hakim — Hadith 4970

It is narrated from Mahmud bin Labid; he said: When the Messenger of Allah (peace and blessings be upon him) set out for Uhud, al-Yaman bin Hisl, the father of Hudhayfah, and Thabit bin Waqsh bin Za'ura' stayed behind in the fortresses with the women and children. These two elderly men said to one another: "May you fare well! What are we waiting for? By Allah, what remains of the life of each of us is no longer than the time it takes a thirsty donkey to quench its thirst; we are only about to depart. Why should we not take up our swords and join the Messenger of Allah (peace and blessings be upon him)?" So they joined the army of the Muslims in such a way that no one knew about them. Thabit bin Waqsh was martyred by the polytheists, while the swords of the Muslims unknowingly fell upon Abu Hudhayfah, and they martyred him without recognizing him. Hudhayfah cried out: "My father! My father!" The people said: "By Allah, we did not recognize him," and they were truthful in what they said. Hudhayfah said: "May Allah forgive you, and He is the Most Merciful of those who show mercy." Then the Messenger of Allah (peace and blessings be upon him) intended to pay his blood money (diyah), but Hudhayfah gave that blood money in charity to the Muslims, and this further increased his honor and standing in the heart of the Messenger of Allah (peace and blessings be upon him).
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.