Sahih Bukhari Volume 006, Book 061, Hadith Number 503.
Narrated By Abu 'Uthman : I was informed that Gabriel came to the Prophet while Um Salama was with him. Gabriel started talking (to the Prophet). Then the Prophet asked Um Salama, "Who is this?" She replied, "He is Dihya (al-Kalbi)." When Gabriel had left, Um Salama said, "By Allah, I did not take him for anybody other than him (i.e. Dihya) till I heard the sermon of the Prophet wherein he informed about the news of Gabriel." The sub narrator asked Abu 'Uthman: From who have you heard that? Abu 'Uthman said: From Usama bin Zaid.