The Holy Prophet (sa) would call Hazrat Sa‘d (ra) his maternal uncle.
(Ashab-e-Badr, Qazi Muhammad Sulaiman Salman Mansur Puri, p. 91, Maktabah Islamiyyah, Lahore, 2015)
On one occasion, Hazrat Sa‘d (ra) was approaching and on seeing him, the Holy Prophet (sa) said, “My uncle approaches; how great indeed is he! If anyone has an uncle the likes of mine, let him show us.”
Imam Tirmidhi has stated the reason for this was that just like the Holy Prophet (sa)’s mother belonged to the Banu Zuhra tribe, so did Hazrat Sa‘d bin Abi Waqas’ (ra) mother.
(Jami‘ al-Tirmidhi, Kitabul Manaqib, Bab Manaqib Abu Ishaq Sa‘d bin Abi Waqas (ra), Hadith 3752)