Malik related to me from Rabia ibn Abd ar-Rahman that az-Zubayr ibn al-Awwam bought a slave and set him free. The slave had children by a free woman. When az-Zubayr freed him, he said, "They are my mawali." The man argued, "They are the mawali of their mother. Rather, they are our mawali." They took the dispute to Uthman ibn Affan, and Uthman gave a judgement that az-Zubayr had their wala'.
USC-MSA web (English) reference: Book 38, Hadith 21
Arabic reference: Book 38, Hadith 1486