On seeing Sa‘d coming, the Prophet (may Allah's peace and blessings be upon him) said: This is my maternal uncle. Is there anyone who has an uncle like him?

Sa‘d ibn Abi Waqqās approached a gathering of the Prophet (may Allah's peace and blessings be upon him) and when the Prophet saw him, he said: This is my maternal uncle, I am boasting about him. Let any person show me his maternal uncle, to show that no one has a maternal uncle like mine. Sa‘d was from Banu Zuhrah clan, and the mother of the Prophet (may Allah's peace and blessings be upon him) āminah, was from Banu Zuhrah as well.

So he was a relative of Āminah, and the relatives of the mother are referred to as maternal uncles.