Hadith: Have you not known that Mujazziz has recently looked at Zayd ibn Hārithah and Usāmah ibn Zayd and said: Indeed, these feet are related to one another?

عن عائشة رضي الله عنها : أن رسول الله صلى الله عليه وسلم دخل عليَّ مسرورًا تبرُقُ أسارِيرُ وجهه. فقال: ألم تَرَيْ أن مُجَزِّزًا نظر آنفًا إلى زيد بن حارثة وأسامة بن زيد، فقال: إن بعض هذه الأقدام لمن بعض». وفي لفظ: «كان مجزِّزٌ قائفًا».
[صحيح] - [متفق عليه]

‘Ā'ishah (may Allah be pleased with her) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) entered upon me with his face shining with delight and said: "Have you not known that Mujazziz has recently looked at Zayd ibn Hārithah and Usāmah ibn Zayd and said: 'Indeed, these feet are related to one another?'" Another wording of the Hadīth adds: “Mujazziz was a lineage tracker.”
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Zayd ibn Hārithah was white, and his son Usāmah was black, and people used to doubt their relation to one another due to their difference in color, suspecting the validity of Usāmah’s attribution to his father. This used to annoy the Messenger of Allah (may Allah's peace and blessings be upon him). Once, Mujazziz al-Mudliji, a lineage tracker, passed by them (Zayd and Usāmah), while their heads were covered with a piece of velvet and their legs were uncovered. On that, Mujazziz said: “Indeed, these feet are related to one another,” due to the resemblance that he saw between them. His speech was heard by the Prophet (may Allah's peace and blessings be upon him) who was extremely happy on hearing that, to the extent that he entered upon ‘Ā'ishah with his face glittering with joy and rejoice for being reassured of the validity of Usāmah’s attribution to his father's lineage; and for the rebuttal of the speech of those who unknowingly set their tongues loose regarding the honor of others.