Sahih Bukhari
Virtues and Merits of the Prophet (pbuh) and his Companions
Volume: 004 / Book Number: 056 / Hadith Number: 755
Narrated 'Aisha:
That Allah's Apostle came to her in a happy mood with his features glittering with joy, and said, "Have you not heard what the Qaif has said about Zaid and Us-ama? He saw their feet and remarked. These belong to each other." (i.e. They are father and son.)