Who is intended by the Prophet's saying: "This is my maternal uncle; let a man show me his maternal uncle," and what was the occasion?

Al-Tirmidhi and Al-Hakim narrated from Jabir that the Prophet, peace and blessings be upon him, said about Sa'd ibn Abi Waqqas: "This is my maternal uncle; let any man show me his maternal uncle." Ibn Hajar mentioned that Sa'd's maternal unclehood to the Prophet, peace and blessings be upon him, was from the side of his mother, Amina, because she was from Banu Zuhra. A similar instance occurred with Abu Talhah Al-Ansari, and Abu Talhah's maternal unclehood to him was from the side of his father's mother, because she was from Banu Al-Najjar.